Could JFK's old seat really fall into Republican hands?

For Democrats, it would be the cruellest stroke of all: the decisive vote against United States health reform, brought about by the loss of the seat held by the legendary American politician most identified with the cause of that reform – and at the very moment when the bruising battle to secure it at last seemed won.

The odds still are the nightmare will not happen. Most observers believe that Martha Coakley will defeat the Republican candidate Scott Brown in the Massachusetts senate election next Tuesday to choose a permanent replacement for Edward Kennedy, who held the seat for almost 47 years until his death last August. But no one any longer is sure, which is startling enough by itself.

Massachusetts, a place virtually synonymous with liberalism in the US, has had the odd Republican governor in recent years. But no Republican has been a senator for the state since Edward Brooke was defeated in 1978, and registered Democrats currently outnumber registered Republicans in the state by three to one.

The loss of the "Kennedy seat," held not only by Edward but by his brother John, the murdered former president, before him, would thus be a huge shock – akin to Republicans losing a Senate seat in Kansas or Texas. Now a contest that once seemed a canter has tightened sufficiently to set Democrats' nerves jangling.

Although a Boston Globe poll at the weekend showed Ms Coakley, the Massachusetts state attorney general, ahead of Mr Brown by a comfortable 15 points, other surveys put her lead in single digits, and one even has her opponent fractionally ahead.

Making Democrats even more anxious, the special election is being held in the depths of winter, on the day after a national holiday, Martin Luther King Day, when many voters may be returning from a long weekend break. Turnout is thus expected to be low, increasing the chances of an upset.

The 50-year old Mr Brown however has also done his part to scramble the odds. A member of the Massachusetts state legislature since 1998, he has been criss-crossing the state in his trademark pick-up truck, selling himself as what some have called a "JFK Republican" – a fiscal conservative and a proponent of a strong national defence, who has served in the National Guard for 30 years, but a moderate on social issues like abortion and same-sex marriage.

He also portrays himself as an outsider, not beholden to the existing Republican establishment. This in turn has boosted his appeal to independents, among whom he is leading by two to one, according to one poll. It is also attracting support from the insurgent Tea Party movement, anti-tax, anti-deficit and ferociously against the emerging health care reform bill, and which is emerging as a force on the national political scene. Mr Brown also opposes the Democrats' proposals as a costly and unwarranted expansion of central government.

Ms Coakley's campaign by contrast has been widely criticised as stale, lacklustre and complacent. In a sign of her newfound concern, she is running attack adverts saying her opponent is "in lockstep" with the unpopular Republican leadership in Washington.

Other television spots seek to link Mr Brown with Sarah Palin, the Republicans' divisive 2008 vice-presidential nominee, little loved in liberal New England. The national party meanwhile is sending some of its biggest guns, including former president Bill Clinton, to stump for Ms Coakley in the closing days of the campaign.

The stakes could not be higher, with health care negotiations in their decisive final phase. The finished product will then have to be voted on by both chambers. Approval in the House is all but certain, whatever the reluctance of liberal Democrats to make concessions to keep conservative Democrats in the Senate on board.

Even then however, the Democrats still require a filibuster-proof super-majority of 60 votes in the 100-seat Senate. Were Mr Brown to win on Tuesday and be quickly seated, the Republicans would gain the crucial 41st vote that would block reform.

The Kennedy: Seat A Democrat domain

So closely associated with American political royalty that it is referred to as "The Kennedy Seat", the Massachusetts senate seat up for election has long been a Democratic certainty. Edward Kennedy, widely considered one of the greatest ever senators, held the job from 1962 until his death last year; before that, his brother John – that is, JFK – had been in place for seven years between 1953 and 1960. When "Ted" died, his nephew Joseph was considered a frontrunner to be his successor – in large part thanks to his family name – until he withdrew from consideration. Even now, Republican candidate Scott Brown is fighting against a natural pro-Kennedy tide. "With all due respect," he said at a recent debate, "It's not the Kennedy's seat, it's not the Democrats' seat. It's the people's seat.