(CNN) - President Barack Obama prepared to head for Massachusetts on Sunday to campaign for the Democratic candidate in a surprisingly close Senate race to fill the seat of longtime liberal lion Ted Kennedy.
Republican state Sen. Scott Brown has surged in recent weeks to catch Democratic state Attorney General Martha Coakley, worrying Obama and Democrats about the possibility of losing what once was considered among their safest seats in the nation.
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 beat Sen. John McCain in Massachusetts last year's presidential race 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 before then.
The Tuesday special election will decide who holds the seat for the rest of Kennedy's term. Democrat Paul Kirk, a long-time adviser and friend to Kennedy, has served as interim replacement since shortly after Kennedy, 77, died of brain cancer in August.
A Suffolk University/7 News poll released Thursday night indicated that 50 percent of likely voters backed Brown, compared to 46 percent support for Coakley. Another 3 percent of people questioned back Joseph Kennedy, a third party candidate running as a libertarian who is not related to the late senator. The poll's sampling error is plus or minus 4.4 percentage points.
Two well-respected non-partisan national political analysts, Charlie Cook and Stuart Rothenberg, have rated the race a tossup.
That is good news for Republicans, who are calling the race a referendum on a national health care bill being pushed through Congress by Obama and Democratic leaders.
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.
"It's extremely close in the most liberal, the bluest state in America," Senate Minority Leader Mitch McConnell, R-Kentucky, said on "FOX News Sunday."
Even if Coakley wins to allow the Democrats to pass the health care bill with their 60-seat filibuster proof Senate majority, the close race shows that most Americans dislike the legislation and want to see it defeated, McConnell said.
"I think the politics are toxic for the government either way," McConnell said of the health care bill. "Either way, whether it passes or it fails, it will be a huge issue not just in 2010 but in 2012."
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. On Sunday, McConnell refused to speculate on a timetable, saying the election winner should be properly sworn in to reflect the will of the people.
"I'm sure all the lawyers will be looking at this," McConnell said.
Both the Coakley and Brown 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.
Former President Bill Clinton held rallies for Coakley on Friday, while former New York City mayor and 2008 GOP presidential candidate Rudy Giuliani has teamed up with Brown.
On Thursday, 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.
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, Alan Silverleib and Tom Cohen contributed to this report.