Mandel Ngan/AFP/Getty Images

By MICHAEL FALCONE ( @michaelpfalcone )

NOTABLES

• GET-THE-GUNS WEEK BEGINS: President Obama, Vice President Joe Biden and Michelle Obama all have events scheduled this week to advocate for the administration's gun proposals, but even though Senate Majority Leader Harry Reid, D-Nev., moved a comprehensive gun package to the calendar for the Senate to consider in April, it is not clear whether Republicans will let the measure come to a vote. ABC's Arlette Saenz notes that President Obama is scheduled to push for gun control today at the University of Hartford, just 50 miles north of Sandy Hook Elementary School, where 20 children and six educators were gunned down last year. On Tuesday, Vice President Biden is scheduled to host an event with law enforcement officials at the White House, and the First Lady Michelle Obama plans to travel to Chicago on Wednesday to speak about eliminating violence in communities. http://abcn.ws/10KYXnq

• THE TIMELINE: The bill that Senate Majority Leader Harry Reid moved to the Senate calendar does not include the controversial assault weapons ban, but concerns about the universal background check requirement in the plan have placed the bill's future in a holding pattern as some Republicans have voiced concern that it will lead to a national registry of guns. The full Senate could begin consideration of the gun measures as early as this week or potentially the week after, but a group of senators, led by Rand Paul, Mike Lee and Ted Cruz, have threatened to filibuster "any legislation that infringe on the American people's constitutional right to bear arms, or on their ability to exercise this right without being subjected to government surveillance." "We will oppose the motion to proceed to any legislation that will serve as a vehicle for any additional gun restrictions," the senators wrote in a letter to Reid.

• 'AN ISSUE THAT HAS 90 PERCENT SUPPORT': White House senior adviser David Pfeiffer told ABC News' George Stephanopoulos on Sunday that the administration believes the background check portion of the bill will be approved "if people are willing to compromise." "Now that the cameras are off, and they're not forced to look the Newtown families in the face, now they want to make it harder, and filibuster it," Pfeiffer said. "If we have a simple up, or down vote, we can get this done." "This is an issue that has 90 percent support. What the president wants is to sign a strong bipartisan bill that has enforceable background checks. And we can get that done," Pfeiffer said on ABC's "This Week." http://abcn.ws/10KYXnq

MARGARET THATCHER, BRITAIN'S IRON LADY, DEAD AT 87. Margaret Thatcher, the first woman ever to serve as prime minister of Great Britain and the longest-serving British prime minister of the 20th century, has died at age 87. "It is with great sadness that Mark and Carol Thatcher announced that their mother Baroness Thatcher died peacefully following a stroke this morning," Lord Timothy Bell, her former adviser, said today, reports ABC's DAVID WRIGHT. Thatcher had significant health problems in her later years, suffering several small strokes and, according to her daughter, struggling with dementia. But during her long career on the political stage, Thatcher was known as the Iron Lady. She led Great Britain as prime minister from 1979 to 1990, a champion of free-market policies and adversary of the Soviet Union. Many considered her Britain's Ronald Reagan. Indeed, Reagan and Thatcher were political soul mates. Reagan called her the "best man in England" and she called him "the second most important man in my life." ABC NEWS OBIT AND VIDEO: http://abcn.ws/10MKaWP

THE ROUNDTABLE

ABC's RICK KLEIN: President Obama today aims for the emotional core of the gun debate, with a visit to Connecticut. The push continues Wednesday with First Lady Michelle Obama headed to Chicago. They'll need every ounce of passion they can muster, and no one questions their personal commitment to seeing gun-control reforms in Sandy Hook's wake. But the White House will be under pressure to show that something is capable of changing in the nation's calcified debate over guns. White House aides may be right about the polling, and right that this represents the best chance in decades to tighten gun laws. That doesn't mean anything will necessarily move, though - or that all the public pressure the president is bringing to bear ultimately matters at all.

ABC's SHUSHANNAH WALSHE: It's the one thing everyone is talking about: Will Hillary run? The Sunday shows, Maureen Dowd, even Tina Brown in her introduction of the former Secretary of State on Friday at her Women in the World Conference said, "The big question for Hillary now is what's next?" The crowd went wild with even Meryl Streep leaping from her seat to cheer. So, what's next? Of course we don't know and this will be a topic of conversation for quite some time, but in what could be read by her supporters as a tantalizing hint, on Friday Clinton said she "looks forward" to being "a partner" in the fight "in all the days and years ahead." Man of her supporters are already convinced she's in it. I spoke to the small group outside of Lincoln Center last week holding "Ready for Hillary" signs and one of them told me, "She's running, she's running. She's just playing hard to get." If she wasn't running, this backer said, Clinton would tell her supporters even this far out not to "waste their energy." Of course every group passionate about their candidate thinks they would call off the troops if they had made a decision, but that's not realistic. Regardless, we will be having this same conversation for quite awhile and, in reality, Clinton may have not made up her mind yet. http://abcn.ws/17mCssM

ABC's GARY LANGER: Margaret Thatcher was broadly popular in the United States, even more so than some of our most celebrated homegrown political figures. At the time of her resignation, in November 1990, 76 percent of Americans in a Gallup poll reported a favorable opinion of her performance as prime minister. For comparison, her compadre Ronald Reagan left office in January 1989 with a job approval rating of 64 percent in ABC News-Washington Post polling. Earlier, in 1984, 84 percent of Americans expressed a favorable opinion of Thatcher personally, again beating Reagan's best in ABC News-Post polling during his presidency.

WHAT WE'RE READING

"TEXAS TURNS BATTLEGROUND AS COWBOY BOOTS WIN HISPANICS," by Bloomberg's Michael Tackett. [Alex Steele] is talking to a group of about 50 activists sitting on plastic folding chairs in Room 101 of the Killeen Community Center, about 70 miles north of the state capital in Austin. Steele is the field director for Battleground Texas, a group that is the offspring of the president's data-driven grassroots organization that many credit with securing his second term. Local Democrats bring a sheet cake that says "Welcome Battleground Texas. Game on Killeen." Steele, 31, and others have come to Texas on a mission as large as the state's 261,000 square miles: to capitalize on the surge in Hispanic population and turn the Lone Star State into a two-party competitive one instead of the place where the Republican nominee has carried every presidential election since 1976. … Population trends are on his side, even if the history of the last quarter-century is not. The transformation underway in Texas is one of many that exemplify how changing demographics are shifting the nation's political makeup as well. In addition to becoming less white, it is also less rural, with 96 of its 254 counties losing population as urban areas grew." http://bloom.bg/Y8lqfE

BUZZ

DAN PFEIFFER: PRESIDENT OBAMA WON'T ENACT 'ROMNEY ECONOMIC PLAN.' Reacting to an early rejection of the expected details of the White House's budget proposal by Speaker John Boehner, President Obama's senior adviser Dan Pfeiffer pushed back Sunday morning on "This Week," saying that the president would not enact "the Romney economic plan," notes ABC's BENJAMIN BELL. "What this president will not do is, come in, right after getting re-elected, and enact the Romney economic plan, which is what the Republicans in the House are proposing," Pfeiffer said. The White House is expected to release details of a budget proposal this week that includes cuts to Social Security and Medicare, which are unpopular among some Democrats. But Pfeiffer said those entitlement cuts would only happen on two conditions. "One, it's part of a balanced package that includes asking - closing tax loopholes that benefit the wealthiest, and two, that it has protections for the most vulnerable, including the oldest seniors," he said. The news of those details in President Obama's budget was met with a swift statement from the House speaker indicating that raising taxes was a non-starter. http://abcn.ws/16G4f6l

ARIANNA HUFFINGTON: HILLARY CLINTON IS 'OBVIOUSLY RUNNING' IN 2016. Editor-in-chief of The Huffington Post Media Group, Arianna Huffington, told ABC's GEORGE STEPHANOPOULOS that Hillary Clinton is definitely in the race for president in 2016. Here's an excerpt of their exchange about Clinton's political future on the "This Week" roundtable: HUFFINGTON: "She's obviously running. But what I was hoping is that she would have taken to more time to become what she called herself, untired." STEPHANOPOULOS: "Untired." HUFFINGTON: "Untired. That was her term, you know? She wanted to sleep in. To be able to recharge herself. She hasn't given herself that time. And I think that's sending a bad message to women that the only way to succeed, the only way to run is to drive yourself into the ground. After all she collapsed, she had a concussion. And right now you see a greater debate among corporations among corporations, among individuals, how can redefine success? How we can actually reduce stress and burnout? Which is having a terrible impact on our health care system." More highlights from the roundtable: http://abcn.ws/YEAWz0

NOTED: HUFFINGTON'S PET PEEVE: PEOPLE BRAGGING ABOUT WORKING 24/7. Huffington told ABC's Benjamin Bell that her biggest pet peeve is "people who pride themselves at working 24/7." "You know, people who say, 'Oh, I don't need sleep,' 'I can just work around the clock.' And they use this, especially men, as a virility symbol, you know. I was having dinner with a guy recently and he bragged that he only got four hours of sleep the night before. And I didn't say it, but I thought to myself, 'You know what, if you had gotten five this dinner would have been a lot more interesting.' So I think the fact that people brag about how they over-work and they don't notice that this is making them less healthy and less effective is a pet peeve." Huffington sat down for a web exclusive interview on Sunday, answering viewer questions from Facebook and Twitter, after her appearance on the "This Week" roundtable. She responded to a variety of questions regarding her perspective as a Greek-American, the Huffington Post news site as well as her Saturday Night Live doppelganger and her "real life" love for Seth Meyers. WATCH: http://abcn.ws/143zvym

CHUCK SCHUMER: IMMIGRATION PLAN WILL BE READY THIS WEEK. As the Senate returns from recess this week, Sen. Chuck Schumer, D-N.Y., said he thinks the bi-partisan Gang of Eight will have its immigration plan completed by the end of the week, ABC's ARLETTE SAENZ notes. "We hope that we can have a bipartisan agreement among the eight of us on comprehensive immigration reform by the end of the week," Schumer said today on CBS' "Face the Nation." "Over the last two weeks, we've made great progress. There have been kerfuffles along the way, but each one of those, thus far, has been settled." Schumer said that the staffs of each Gang of Eight member has worked 12 hours a day to fine-tune the details of their immigration plan and reach an agreement on every issue. In an interview on NBC's "Meet the Press," Sen. Lindsey Graham, R-S.C., also a member of the Gang of Eight, set a longer time frame of a "couple of weeks" before the plan is completed. But one Republican member of the bi-partisan group has expressed concern that the deal on immigration reform is being reached in haste. Sen. Marco Rubio, R-Fla., has called for more hearings and time to review the plan in order to encourage "healthy public debate." http://abcn.ws/12wHCCC

TOP U.S. GENERAL IN SOUTH KOREA CANCELS TRIP DUE TO NORTH KOREA SITUATION. The ongoing tense situation on the North Korean peninsula has led the top American general in South Korea and South Korea's top general to cancel long-planned trips to Washington this week, reports ABC's LUIS MARTINEZ. It has also led U.S. Defense Secretary Chuck Hagel to postpone an ICBM test launch scheduled for Tuesday to avoid a" misperception or manipulation" of the test by North Korea. On Sunday, Col. Amy Hannah, a U.S. Forces Korea spokesperson, said that Gen. James Thurman, the commander of U.S. and U.N. forces in South Korea, would not be traveling to Washington this week for previously scheduled congressional budget hearings. "Given the current situation General Thurman will remain in Seoul next week as a prudent measure. He has asked the Senate Armed Services Committee," said the statement. http://abcn.ws/Z1p3Tm

WHO'S TWEETING?

@David_Cameron: It was with great sadness that I learned of Lady Thatcher's death. We have lost a great leader, a great Prime Minister, and a great Briton.

@SenJohnMcCain: RIP Margaret Thatcher, one of the great leaders of the 20th century

@jonkarl: "Defeat? I do not recognize the meaning of the word" - Margaret Thatcher, 1982.

@SteveRattner: You can be a Democrat and still believe that Margaret Thatcher was right for Britain (and the world) in the 1980s.

@KellyO: Farewell to the Iron Lady. UK's only woman to serve as Prime Minister. Without Thatcher, would a woman US president seem so possible?

@RichardHaass: just learned Margaret Thatcher died; remember her as principled, strong-willed, always willing to hear you out partner during 90/91 Gulf war

@MayorofLondon: Very sad to hear of death of Baroness Thatcher. Her memory will live long after the world has forgotten the grey suits of today's politics.

@kelleymc: One of my 1st childhood memories of a powerful political woman MT @KattyKayBBC Baroness Thatcher dominated British politics in the 80s

@BeschlossDC: Here at start of their friendship R. Reagan writes M. Thatcher after 1st private meeting & fall of Saigon 4/30/1975: pic.twitter.com/uFIBoy37xe