It started out as a run-of-the-mill Republican primary. But Pat Toomey's challenge for incumbent Arlen Specter's Senate seat in Pennsylvania has turned into a civil war between the party's centrist and conservative wings, in which control of the Senate, and even of the White House, could be at stake.
A week before Tuesday's vote, the two contenders are criss-crossing the state, setting out their very different positions, in a contest which has dragged in President George Bush.
For the moderate Mr Specter, who is seeking a fifth six-year term in the Senate, his upstart conservative rival - currently a sitting Congressman - is "not far right, he's far out". To which Mr Toomey, pro-life, anti-tax and an ardent de-regulator, replies by calling himself the candidate of "the Republican wing of the Republican party".
The race, according to a new survey, could hardly be closer. Only 10 days ago Mr Specter enjoyed a massive financial advantage and a comfortable 15 per cent lead in the polls. The margin has now shrunk to 5 per cent among likely Republican primary voters, in statistical terms a virtual dead heat.
So much importance does the White House attach to the result that Mr Bush turned out for a Specter fundraiser in Pittsburgh on Monday. But the momentum is with Mr Toomey. While Mr Specter has been forced to spend $7m (£3.9m), half his total campaign war chest, his opponent has been raking in money from ultra-conservative Republican groups.
These efforts appear to be paying dividends. In the new poll, 51 per cent of respondents described Mr Specter as "too liberal", while only 14 per cent said they considered Mr Toomey too conservative.
The struggle is being watched with anxiety by Republican moderates in the Senate - already something of an endangered species as US politics have grown increasingly polarised. As a result, the Republicans have grown more conservative and the Democrats more liberal, with ever fewer centrists to fashion legislative compromises between them. Defeat for Mr Specter would send an unambiguous message that moderation is a recipe for political defeat.
But the stakes are not just ideological. This November, Republicans will be defending a slim 51-48 majority in the Senate. With his appeal to moderates and independents, Mr Specter would be expected to hold his seat comfortably in November. Mr Toomey would probably have a much tougher fight - although his supporters point out that Pennsylvania's other Senate seat is held by another out-and-out Republican conservative, Rick Santorum.
The outcome could have a bearing on the presidential vote as well. With its 21 electoral college votes, Pennsylvania - which Al Gore narrowly won in 2000 - is top of the Republican list of target states this time around. Party strategists believe Mr Bush would have a better chance of capturing it if a centrist like Mr Specter also features on the Republican ballot. For that reason, Mr Bush took time out to endorse Mr Specter in person at this week's fundraiser.
No matter that the two have frequently been at odds on policy, notably over Mr Specter's demands for stronger civil liberties protection in the anti-terrorism Patriot Act, rejected by Mr Bush. No matter that among Mr Specter's supporters is the billionaire philanthropist George Soros, who has already devoted $12m of his fortune to help Democrats drive Mr Bush from the White House.
Yes, the 74-year old senator was "a little bit independent-minded sometimes", Mr Bush declared. But there was nothing wrong with that. "Arlen Specter is a fine legislator and the right man for the United States Senate." The Senator responded by swallowing his objections to the Patriot Act, praising the President's "tremendous leadership" in the war on terrorism.
And even for this most conservative President, Mr Toomey is probably too much of a good thing. His biggest financial backer is the Club for Growth, an anti-tax group which has criticised Mr Bush's substantial tax cuts as inadequate, and has fallen foul of Karl Rove, the President's powerful political adviser.Reuse content