In a huge boost for Barack Obama and his reformist programme, the veteran Pennsylvania Republican Arlen Specter announced yesterday he was switching to the Democrats.
The shock move underlines the growing marginalisation of the Republicans in US politics, and makes it more than likely his new party will soon have a filibuster-proof majority in the Senate.
The immediate reason for Mr Specter's decision, which delighted Democrats as much as it stunned Republicans, was the tough primary challenge the 79-year-old was facing in his bid for a sixth term in the Senate of which he has been a member since January 1981.
But it is another sign of how Republicans have moved so far to the right that many moderates, especially in states like Pennsylvania that have been trending Democratic in recent elections, are abandoning the party. As such, his move could be part of a broad realignment of US politics, evident in the substantial Democratic gains in the 2006 mid-term elections, and even more so in Mr Obama's sweeping victory last November.
Mr Specter has long prided himself on his independence and unpredictability, and there is no guarantee he will vote with his new party on every issue. But the Democrats are now within touching distance of the 60 votes required in the Senate to break Republican filibusters. That power, allied with the Democratic majority in the House, would give them complete control of the political agenda on Capitol Hill.
Currently the Democrats have 56 votes, and can count on two independents who almost always vote with them. A 59th vote has long been likely, in the shape of the former comedian and author Al Franken, whose lead of 312 votes out of 2.9 million cast in last November's Senate election in Minnesota has been upheld by a state court. Norm Coleman, Mr Franken's Republican opponent, is appealing the ruling to Minnesota's Supreme Court – but with scant prospect of success, experts say. Now the party has its 60th.
The Specter switch could not be a better 100-day anniversary present for Mr Obama. A liberal Republican, he was one of just three of the party's senators to break ranks and support the President's $780bn (£530bn) economic stimulus package, to the fury of conservatives. As a Democrat, he could be a crucial vote in pushing Mr Obama's ambitious tax, health care, education and energy policies through Congress.
"You have my full support and we're thrilled to have you," Mr Obama told the defecting senator in a phone call, minutes after the President learned the news. Harry Reid, the Democratic majority leader in the Senate, added: "I welcome Senator Specter and his moderate voice to our diverse caucus." From shellshocked Republicans, the initial reaction was numbed silence.
Mr Obama's words mean that the powerful Democratic party machine in Pennsylvania – a state the President carried by 11 points in November – will now throw its weight behind Mr Specter. No less important, the latter will now sidestep a Republican primary challenge from the conservative Pat Toomey, who came close to an upset in 2004 when Mr Specter last ran for re-election. Polls have suggested Mr Toomey might win this time, and Mr Specter himself had publicly acknowledged he was facing his toughest political battle since joining the Republican party in 1966.
Even more alarming for Republicans, the defection is evidence of how the party is losing relevance in swathes of the country, especially the North-east, as its moderate wing has shrivelled.