Polls have closed in Massachusetts for a hotly contested special election to decide who will take the Senate seat held for 46 years by the late Edward Kennedy.
Hanging in the balance is what the outcome could mean for health care overhaul and President Obama's overall agenda, because if Republican Scott Brown defeats Democrat Martha Coakley, it would mean the end of the Democrats' super majority in the Senate.
Brown, 50, is in a surprising neck-and-neck race with Coakley, 56, even though just a few weeks ago most polls showed Coakley with a solid lead.
The last time a Republican senator was elected in Massachusetts was November 1972, when Edward Brooke won the seat. And given the history of Massachusetts' voting pattern, Brown's popularity is a surprise to many.
Tonight, a lawyer for Coakley alleged there were reports from voters of ballots being pre-marked with a vote for Brown, possibly laying a foundation for a vote recount.
"Let there be no doubt, those are spoiled ballots, they should not be counted. And they should be preserved by the local election officials. We have brought it to the attention of the Secretary of State's office," said Democratic lawyer Marc Elias.
"We just want to make sure for the people going forward for the remainder of the day take the time and they don't just walk out without voting, or cast the ballot that's been pre-marked," he said.
The White House acknowledged today that Obama "was both surprised and frustrated" by how hotly contested the special election has become. Press Secretary Robert Gibbs told reporters today that the president is "not pleased" with how much Coakley is struggling.
But when asked if Obama believes health care overhaul efforts will die if Brown wins, Gibbs said, "Let's wait for the results. I don't think the president believes that."
Gibbs said that no matter what happens in Massachusetts tonight, "we face a set of circumstances that have to be addressed and have to be dealt with" and there isn't a new agenda that will be based on tonight's results.
If Brown is elected, Senate Democrats would lose the 60-seat majority they now enjoy and that they need to avoid a Republican filibuster of the health care bill.
In case of a Democratic loss, sources said, the White House wants the House to vote directly on the same bill the Senate passed Christmas Eve which would avoid the need for a conference to reconcile the two bills. It would prevent Brown from having any chance to cast a vote on health care.
Administration officials argue that if it's not the Senate bill, then there may not be any health care overhaul. But with some moderate Democrats on the sidelines, there would likely not be enough votes to pass the Senate bill.
"To be honest with you, that's going to be a very hard sell for everybody," Rep. Joe Courtney, D-Conn., said Monday in an MSNBC interview. "And it's not just a Progressive Caucus issue. It really would require a lot of trust to say that there would be a follow-on to incorporate some of these changes."
Today, Speaker of the House Nancy Pelosi, D-Calif., effectively denied that any talk of avoiding reconciliation was taking place. Instead, she said members of Congress are moving forward with reconciling the Senate and House bills and wouldn't say if there is a back-up plan should Brown win.
"Regardless of what happens in Massachusetts, we still have to resolve the differences between our two bills," Pelosi told reporters today. "Our eye is on the ball of passing legislation. In order to do that, we have to resolve some differences, establish some priorities, make some decisions."
Pelosi, who has visited the White House several times in the past few weeks to discuss health care, said today that Democrats would move ahead with their legislation regardless of what happens in Massachusetts.
"Whatever happens in Massachusetts, we will have quality affordable health care," she said.
Rep. Steny Hoyer, D-Md., was asked today that if Brown wins and assuming the maximum amount of time to certify the election is 15 days, is it feasible to pass legislation the magnitude of health care reform in the next 15 days, to which the House majority leader replied, "Yes."
Other Democrats also echoed Pelosi's comments.
"We might not get the biggest package, but I can tell you this much. Democrats will continue to fight to make major changes in our health care system," Democratic strategist Donna Brazile said on "Good Morning America" today.
Even some Republicans doubt that the momentum on health care overhaul would die down completely. Republican strategist Mary Matalin said on "GMA" today that Brown's win wouldn't necessarily mean that health care is dead. But it would mean that members of Congress have to start from scratch.
"If he's in there, we can go back to the drawing board and we can get those kinds of reforms that people want," Matalin said.
Even if Brown were to win, Democrats would still have the largest Senate majority either party has enjoyed since 1979. They just won't have the 60 votes to avoid a Republican filibuster.
One idea House Democrats were discussing, assuming Brown wins, is having Senate Democrats force the bill through by bypassing normal Senate rules and passing the legislation through reconciliation, which requires only 50 votes.
But sources say that idea is off the table, because it would mean they have to start over and would also risk losing some moderates, and that could cost Democratic leaders the health care legislation.
Americans' views on health care have stayed stagnant since August.
In an ABC News/Washington Post poll released today, 51 percent of Americans said they oppose health care overhaul efforts, with only 44 percent in favor.
At its peak, in September and again in November, 30 percent of Americans "strongly" backed the proposed changes. With the plan still undergoing modifications, that has dropped to 22 percent, a new low. Substantially more, 39 percent, are strongly opposed, a number that's held steadier than the percentage of those who support the proposed changes.
Election Draws Large Crowd in Massachusetts
Special elections do not tend to draw large crowds, but despite predictions of severe weather in some parts of the state, high voter turnout is expected.
"It's a good turnout. People are coming out," said a spokesman for William Francis Galvin, the top election official in Massachusetts.
Galvin, the secretary of the commonwealth, expects the turnout to be between 1.6 million and 2.2 million and he considers it good because this is a statewide, stand-alone, special election, and the Senate seat is the only contest on the ballot.
But the turnout projection is still well below the turnout during the November 2008 election between Barack Obama and Sen. John McCain. At that time, 3.1 million voters cast their ballots and Obama won in every county in Massachusetts.
Polls will close today at 8 p.m. ET.
Many independents, the biggest voting block in Massachusetts, were turning to Brown today, even though they voted for Obama in 2008.
"Listening to him, I feel like he put on an awesome campaign," Massachusetts resident Annemarie Gibbs told ABC News.
Brown's popularity has largely emerged from his appeal to Independents, the same group that Obama touted as a presidential candidate. On the ground, he hit the trail with gusto, making his pickup truck the center of his campaign and tapping into voter anxiety and frustration over the economy.
"Down to earth guy, he's walking around the neighborhoods," Randy Suchniu told ABC News.
Online, he took a page out of Obama's playbook, with his supporters swarming the Internet. He has five times as many fans on Facebook as Coakley, twice as many twitter followers and google searches running in his favor by 2-1. Brown made $1 million a day every day last week.
It's not like Coakley hasn't enjoyed any support. The seasoned state politician has been in the public eye for quite some time.
"She has always been proven to get her hands dirty," resident Michael Otero told ABC News.
But even her fans say they would've liked to see more from her.
"I wish she was more aggressive," Otero said.
Casting her vote today, Coakley expressed cautious optimism to reporters.
"We're paying attention to the ground game. ... Every game has its own dynamics," the state attorney general said. "We'll know tonight what the results are."
Brown played down the impact his potential win could have on the health care vote.
"It would make everybody the 41st senator, and it would bring fairness and discussion back to the equation," he told reporters today after voting.
Brown has said he is an independent Republican, although he would not say how he would vote differently from Republicans in Washington.
"I'm not going to focus on what's happened in the past," he said. "I'm going to focus on what I have an ability to deal with in the future."
There are two things in this race that Democrats and Republicans can both agree on. One, they have never seen a Senate election this intense in Massachusetts, and, two, the stakes could not possibly be higher.
Massachusetts Special Election
Even though he is not on the ballot, Obama faces the prospect of a significant defeat if Brown wins, a reality for which Democrats have braced themselves.
And it's not just health care that worries Democrats. Brown's win could put Obama's entire agenda at risk, including his push on climate change, regulatory legislation and banking industry overhaul. There has so far been little or no bipartisanship on such major issues.
Obama himself acknowledged that in his stump speech for Coakley.
"A lot of these measures are going to rest on one vote in the U.S. Senate," the president said in Boston Sunday.
In an e-mail pitch, the president today urged the party's Organizing for America supporters to hit the phones for Coakley.
"The polls are still open, the choice has not been made and you still have a crucial role to play by calling voters in Massachusetts," he wrote. "In a low-turnout special election like this one, every single voter counts. In a race as close as this one, no matter how many voters you call, you could tip the balance."
In a race that has pulled in the president and Democrats and Republicans from across the country, Coakley's supporters worked the phones up until the last minute. She even unveiled a TV ad Monday featuring Obama's pep rally for her.
Brown also worked the crowds. His popularity in part has been because he has tapped into voter anger and anxiety over the economy.
"I think this race has really stirred the passions of people across the country," Brazile said on 'GMA' today. "This is a race that matters to Democrats across the country because the president's domestic agenda really requires Democrats across that state to come out today, to show their support and back this candidate."
Some Democrats are still pinning their hopes on Coakley, saying that her last-minute campaign push could turn the tide.
"Scott Brown has run a remarkable campaign for a Republican in the Bay state, but there's no question that the Democrats are now anxious to win this seat," Brazile said. "I think Democrats will churn out their voters and shock the Republicans that we're not going to send another no-vote to Washington, D.C."
If he wins, Brown, a lawyer and former model, could put the Democrats' health care agenda at risk. Republicans said that might not be such a bad thing.
Matalin said that for Obama, it's been "a year of living dangerously."
"He has driven his party into a brick wall on all of these issues across the board," she said, and compared to Clinton, who Matalin said paid attention to what Americans wanted, "this one seems determined to do quite the opposite."
ABC News' Jake Tapper, Jonathan Karl and Teddy Davis contributed to this report.