(CNN) - Republicans may be on the verge of pulling off what was, until now, politically unthinkable: a GOP win in the race to fill longtime liberal lion Ted Kennedy's U.S. Senate seat.
Tuesday's special election is now deadlocked, according to a new poll.
President Barack Obama will campaign in Massachusetts Sunday to try to help save the seat for the Democrats, White House spokesman Robert Gibbs said.
A GOP victory in the overwhelmingly Democratic state could give Senate Republicans enough votes to block Obama's health care plan. It could also shatter assumptions about the competitiveness of politics in the progressive northeast.
No Republican has a won a U.S. Senate seat in Massachusetts since 1972.
The state's entire congressional delegation is Democratic. Obama crushed Sen. John McCain in Massachusetts last year, beating the GOP nominee by 26 points. Kennedy - an advocate for liberal health care reform throughout his career - held his seat for over 46 years. His brother, President John F. Kennedy, held it for another eight.
A Suffolk University/7 News poll released Thursday night, however, indicates that 50 percent of likely voters now back GOP state Sen. Scott Brown.
Only 46 percent support Democratic nominee Martha Coakley, the state attorney general.
Three percent of people questioned back Joseph Kennedy, a third party candidate who is not related to the late senator.
Brown's 4-point lead is barely within the survey's sampling error.
"It's a massive change in the political landscape," said David Paleologos, director of Suffolk's Political Research Center.
A Boston Globe poll released last weekend indicated Coakley had a 15-point lead over Brown, but other recent surveys, including several partisan polls, suggested the race was much closer.
Thursday afternoon two well-respected non-partisan national political analysts, Charlie Cook and Stuart Rothenberg, rated the race a tossup.
According to the Suffolk University survey, Brown is grabbing 65 percent of independent voters, with just three in 10 pulling for Coakley. And 17 percent of Democrats questioned say they're supporting Brown.
If Brown pulls an upset and defeats Coakley, the Democrats will lose their 60-seat filibuster-proof coalition in the Senate. The shift could severely threaten the party's priorities on health care and a range of other
issues. Brown has promised to vote against the health care bill if elected.
Kennedy was 77 years old when he died of brain cancer last August. Democrat Paul Kirk, a long-time adviser and friend to Kennedy, is serving as his interim replacement.
Massachusetts Secretary of State Bill Galvin, a Democrat, said this week that certifying Tuesday's election results could take more than two weeks - potentially enough time to allow congressional Democrats time to pass a final health care bill before Brown is seated.
Republicans blasted the timetable, accusing Democrats of trying to stifle the will of the voters.
The Suffolk poll indicates that 51 percent of voters oppose the health care reform legislation, with more than six in 10 saying they believe the government can't afford to pay for it.
The Suffolk survey was conducted Monday through Wednesday, with 500 people who said they are likely to vote in the January 19 special election questioned by telephone.
The poll's sampling error is plus or minus 4.4 percentage points. Both campaigns have raised large sums of cash in the past week and combined have spent well over a $1 million on television commercials. National party organizations as well as independent groups have also flooded the airwaves in the typically politically uncompetitive Massachusetts.
Aside from Obama, two well-known out-of-state surrogates are now teaming up with the candidates. Former President Bill Clinton is holding rallies in Boston and Worcester for Coakley on Friday, while former New York City mayor and 2008 GOP presidential candidate Rudy Giuliani teams up with Brown.
On Thursday, however, Obama put out an e-mail and Web video in support of Coakley. The e-mail was sent to the Massachusetts distribution list of Organizing for America, the president's political arm at the Democratic National Committee.
"I am supporting Martha Coakley. As your attorney general, she's taken on Wall Street schemes, insurance company abuses, and big polluters," Obama says in the e-mail. "She'll be your voice, and my ally. And she needs your help."
Vicki Kennedy, the late senator's widow, also is making a pitch for Coakley in a new television campaign commercial that started airing Friday.
–CNN's Paul Steinhauser and Alan Silverleib contributed to this report