J. Scott Applewhite/AP Photo

By MICHAEL FALCONE ( @michaelpfalcone )

NOTABLES

• A DAY OF RECKONING: Today, consideration of the Manchin-Toomey background check proposal and a myriad other gun amendments, including a ban on assault weapons and high-capacity clips, will be the main event on Capitol Hill, ABC's SUNLEN MILLER reports. The outcome of these votes will determine the fate of the biggest gun control legislation the Senate has seen in two decades. The vote on the Manchin-Toomey compromise will be razor thin and with last night's announcement by one of the few remaining undecided Republicans - Sen. Lisa Murkowski - that she would not vote for it, it looks increasingly difficult for the bill to garner the 60 votes.

• THE UNDECIDED: There are now just two Republican Senators still undecided - Sen. John McCain, R-AZ., Alaska and Kelly Ayotte, R-NH. There are four Democrats still undecided Sens. Mary Landrieu of Louisiana, Mark Begich of Alaska, Heidi Heitkamp of North Dakota and Mark Pryor of Arkansas. A wild card is Sen. Frank Lautenberg, D-New Jersey., who, although he supports the amendment, has been ill and home in NJ. Aides say the Senator "hopes" to get back for the vote today.

• TIMING: The Senate convenes at 9:30 a.m. Senate Majority Leader Harry Reid said yesterday that the Manchin-Toomey amendment is set to come up for a vote at 4 p.m., followed by at least eight other amendments, according to ABC's JEFF ZELENY.

• THE NRA'S PLAY: The National Rifle Association is out with a last minute Internet ad today citing statistics that police do not agree with the gun proposals being promoted by President Obama and New York City Mayor Michael Bloomberg, reports ABC's ARLETTE SAENZ. "President Obama and Mayor Bloomberg are pushing gun control but America's police say they're wrong," the ad says before listing off statistics. The ad will run on the internet with a $500,000 buy starting today. It will appear on sites like the Drudge Report, the Washington Post, Hulu and other Beltway publications. http://bit.ly/13iSDEc

THE ROUNDTABLE

ABC's JONATHAN KARL: I expect the Senate today to reject the one gun proposal that had a chance of passing. Supporters of the Toomey-Manchin background check compromise acknowledge that it will not have enough votes to pass. This is a big defeat for President Obama, who yesterday was making calls to red-state Democratic Senators pleading for their votes.

ABC's RICK KLEIN: Now what? Barring a shocking turnaround (actually, several), the celebrated Toomey-Manchin background checks compromise will meet a grim end this afternoon in the Senate. It was never going to fly in the House, anyway, without a resounding vote in the Senate. So the question for gun-control advocates - including the Newtown families whose success on the Hill was being celebrated last week - will become whether something, even something extremely minimal, is better than nothing. Does the momentum for gun control shift behind any of the Republican proposals, which include some strengthened background checks (though not expanded checks) but are paired with gun-rights expansions? Or does the failure mark the end of a months-long emotional lobbying push that few thought was going to change the dynamics of gun-control laws when it started?

ABC's JIM AVILA: So eight senators have agreed on a "pathway to citizenship" for America's undocumented, leaving a mere 92 to go. And make no mistake, there are many stumbling blocks on this road to legalization. Pro-pathway advocates are concerned that too many people are being left out, because anyone here after December 2011 will not be eligible for citizenship and will still be vulnerable to deportation. For many on the other side, the border will never be secure enough, even if this new immigration reform bill does call for $4.5 million in funding for new border patrol agents, equipment including drones and more security fencing. Plus history is working against the bill. A similar version failed in 2007 because of conservative outcry. According to a newly released ABC News/Washington Post poll, Americans were evenly split on the issue (49 percent to 46 percent) in 2007, whereas today that number is 63 percent to 33 percent. That may mean the difference between the 2012 bill's passage and the 2007 bill's demise. Schumer said the fact that labor is on board now, when in 2007 they were most definitely not, means this bill has a greater chance of success. http://abcn.ws/Zznbxq

ABC's Z. BYRON WOLF: Is John McCain becoming the GOP's Teddy Kennedy? It has been 24 years since John McCain nearly had to resign from the Senate amid scandal. It has been 8 years since a presidential drubbing kept him from being promoted. But he has found a way to overcome both and to remain at the center of every major Senate debate. Smart people say if he supports a struggling compromise background check bill for gun purchases, it could give other waffling Republicans cover to jump onboard too. He is also at the center of a fragile immigration reform compromise, reprising a role he played years ago with Sen. Teddy Kennedy, the Massachusetts liberal who 2009. You could argue McCain has become like a Republican version of the liberal lion. Both men survived scandal and both had higher ambitions quashed. But both gained a sort of prominent tenure on Capitol Hill.

ABC's CHRIS GOOD: The word "terrorism" is now officially attached to the Boston Marathon bombing, after President Obama used the word Tuesday morning in his address from the White House. "Given what we now know about what took place, the FBI is investigating it as an act of terrorism," Obama said yesterday. What makes something a terrorist event? According to David Gomez, a former FBI counterterrorism official, Justice Department protocol automatically classifies bombings of this nature as terrorism cases, partly for jurisdictional purposes: Until deemed otherwise, the FBI (and not Bureau of Alcohol, Tobacco, Firearms and Explosives) handles bombings as terrorism investigations. "There's a number of different definitions for 'terrorism.' There's the academic definition, and there are the legal definitions, and then there's the one that's used by the FBI, which kind of straddles both," Gomez told ABC News. "The FBI uses a definition that talks about coercing the civilian population or the government … through violence. … It's an attempt to change their minds about certain things." A former senior assistant special agent-in-charge for counter-terrorism and intelligence at the FBI's Seattle Field Office, Gomez said the word "terrorism" carries political connotations about a perpetrator's motive. Newtown, Conn., shooter Adam Lanza, for instance, was not a terrorist because he wasn't trying to communicate a message to the public or the government. http://abcn.ws/11sEHqN

BUZZ

REPUBLICAN SENATOR RECEIVES POTENTIALLY POISONED LETTER. A suspicious letter potentially laced with a poison, ricin, and postmarked from Memphis, was sent to the office of Sen. Roger Wicker of Mississippi, ABC's JEFF ZELENY reports. Senate Majority Leader Harry Reid told ABC News that Capitol offices were on high alert. The FBI was investigating the incident, officials said. Sources confirmed the letter was sent to Wicker, R-Miss., but did not arrive at his office on Capitol Hill. It was stopped at a mail processing facility, where, officials said, a preliminary test for ricin came back positive. Wicker released a statement on the investigation into the letter late last night: "This matter is part of an ongoing investigation by the United States Capitol Police and FBI," Wicker said. "I want to thank our law enforcement officials for their hard work and diligence in keeping those of us who work in the Capitol complex safe. [My wife] Gayle and I appreciate everyone's thoughts and prayers." http://abcn.ws/YuTGxp

WHODUNIT? A senior Senate official told ABC News that authorities had identified and were interviewing a person of interest - someone who frequently writes letters to members of Congress. There were no injuries, but the senior official said the event was being treated as "totally real." Members of Congress were also taking extra security steps at their district offices in their home states. http://abcn.ws/YuTGxp

MAYORS MENINO, GIULIANI PRAISE BOSTON'S 'BRAVE' FIRST RESPONDERS. Boston Mayor Thomas Menino yesterday received condolences for the Boston Marathon bombing from former New York City Mayor Rudy Giuliani, who compared it to the Sept. 11 World Trade Center attack. "One of the images that I take away from yesterday, having watched it all day, were your firefighters, your police officers and some of your citizens running right into the fire, and that reminded me of my firefighters and my police officers who ran into the building," Giuliani told Menino, speaking mayor-to-mayor in a joint "World News" interview with ABC's DIANE SAWYER. Mayor Menino agreed, adding firefighters and EMS to the list of American heroes in Boston Monday. "They reacted instantly and helped save lives," Menino said. "When it blew up, they moved quickly, and I'm very proud of what the first responders did yesterday afternoon." "People of Boston should be, too," Giuliani said. "They should be very proud of their citizens, all of whom reacted in a very, very brave way, and I was very heartened by that." The brief meeting between the two was a surprise for Menino, who did not know Giuliani was listening in on his interview with Sawyer. http://abcn.ws/YRXmOz

GIULIANI'S ADVICE: ABC's SARAH PARNASS notes that former Mayor Giuliani's advice to the people of Boston was to "go about their lives." "They should not let whatever form of terrorist this is, they shouldn't let them win, because this is what they're trying to do. They can't capture us. They can't overwhelm us. They can't destroy us. All they can do is frighten us and try to stop us from doing the things that make us a great country. We shouldn't let them do that," the former New York City mayor said. "My heart goes out to them." http://abcn.ws/YRXmOz

WORD FROM THE WHITE HOUSE: ABC's ANN COMPTON hears from a White House aide: "Overnight, the President continued to receive updates from Assistant to the President for Homeland Security and Counterterrorism Lisa Monaco on the FBI led investigation into the explosions in Boston. In addition to the direct briefings the President has received, senior White House officials have convened multiple briefings with senior members of the federal response and law enforcement team over the last two days - including last night. Later this morning, the President will receive a briefing from Attorney General Eric Holder, FBI Director Robert Mueller, Assistant to the President for Homeland Security and Counterterrorism Lisa Monaco and other senior members of his team."

NOTED: This afternoon President Joe Biden participates in a Google+ Hangout with mayors from across the country to discuss reducing gun violence, including Mayor Karen Freeman-Wilson, Gary, IN; Mayor Stephanie Rawlings-Blake, Baltimore, MD; Mayor R.T. Rybak, Minneapolis, MN; and Mayor Steve Scaffidi, Oak Creek, WI.

OBAMA HEADS TO BOSTON THURSDAY. President Obama is scheduled to travel to Boston Thursday, three days after the deadly bombings at the Boston Marathon, ABC's MARY BRUCE reports. As the city grapples with this tragedy, the president plans to speak at an interfaith service "dedicated to those who were gravely wounded or killed in Monday's bombing," White House Press Secretary Jay Carney said in a written statement. Obama's visit comes as authorities are working around the clock to determine who is behind the twin blasts at the finish line of the marathon. Earlier today, the president vowed to bring to justice those responsible for this "act of terrorism," which he called a "heinous and cowardly act."

WHAT WE'RE READING

-" SCHWARZENEGGER PLANS IMMIGRATION FORUM AS D.C. LEADERS DEBATE PLAN," by The Los Angeles Times' S eema Mehta. "As a bipartisan group of leaders in Washington prepare to unveil a landmark immigration bill, former California Gov. Arnold Schwarzenegger is planning to host a forum at USC focused on the matter that features significant players in the debate. Among the speakers scheduled to participate in the April 30 summit are former Mexican President Vicente Fox, former U.S. Commerce Secretary Carlos Gutierrez, Sen. John McCain (R-Ariz.) and Sen. Michael Bennet (D-Colo.). 'Reforming our broken immigration process demands action,' Schwarzenegger said in a statement to The Times. 'I'm excited to host this forum during a critical time in the immigration debate and believe we have an opportunity to elevate the issue above partisan differences.'"

-" HOUSE DEMS SHOW BETTER FIRST-QUARTER FUNDRAISING THAN AT-RISK REPUBLICANS," by The Hills' Cameron Joseph. "House Democrats are winning the early round of the 2013 fundraising battle over a number of at-risk Republicans. Several House Republicans facing potentially tough reelection campaigns reported lackluster results in first-quarter filings with the Federal Election Commission, which were due Monday. Those stragglers could prove a headache for the National Republican Congressional Committee (NRCC) as it fights to prevent Democrats from picking up seats - or winning back the House - in the 2014 midterms. 'This should be a wake-up call to Republican members,' one GOP strategist told The Hill." http://bit.ly/10eXnxI

IN THE NOTE'S INBOX

-BUSINESS GROUP TOUTS BENEFITS OF IMMIGRATION REFORM. A new report to be released today by the Obama administration-friendly group, Business Forward, argues that "immigration reform for high skilled workers has the chance to create a system that attracts entrepreneurs and could reverse the drop in business starts the country has seen since the recession, which could create new jobs. If employment from brand-new businesses, which are more likely to boost job growth, returned to pre-recession levels, we'd see another 200,000 jobs per quarter." REPORT: http://bit.ly/17kS3X3

WHO'S TWEETING?

@AlexConant: Here's a 1-page summary of the bipartisan immigration reform bill: http://www.rubio.senate.gov/public/?a=Files.Serve&File_id=adefad85-7f5c-4f3e-a4dc-366a1b71ad38 …

@DavidMDrucker: House immigration negotiators hope to unveil bill in May; Labrador supports moving in pieces w/ coordinated votes: http://bit.ly/Zzzmdv

@tedhesson: Reading through the proposed merit-based part of the immigration system now. Things like employment, age, family and educations all count

@JillDLawrence: The meaning of terrorists striking a soft target. Good insights from @ron_fournier @nationaljournal http://bit.ly/XR4gDV #Bostonmarathon

@llerer: The Boston bombing hit close to home for Obama's new counterterrorism adviser Lisa Monaco. Even her mom says so. http://ow.ly/k8phB