Senator Arlen Specter's decision to cross the aisle and join the Democratic Party hastens the extinction of moderate Republicans in the north-east and symbolises their deep problems. But it does not change the legislative landscape.
On the most contentious parts of the President's programme - cap-and-trade emissions programme, healthcare and social security - the key divisions are among Democrats rather than between the parties. Mr Specter's defection numerically swells the party's ranks but in practice brings the administration no closer to the magic 60 votes it needs to push through ambitious reform proposals.
Senate tradition allows even a single senator to block passage of legislation by filibustering it. Since time is precious, even the threat of a filibuster is usually enough to stop legislation in its tracks.
Rule XXIII (cloture) curtails the right of unlimited debate and caps further debate at 30 hours. But it can only be invoked with agreement of 60 senators (three-fifths of the chamber's total membership).
So the real majority required to pass contentious legislation is 60 rather than 51 (50 assuming the vice-president uses his casting vote).
Such 60-vote majorities are rare. The last was in 1978. But the Democratic Party has been inching close. The party has support from 58 senators (56 elected as Democrats, two as independents). Mr Specter's defection will give it 59. If Democrat Al Franken is confirmed as the winner of Minnesota's disputed election by the state Supreme Court, the party will get the 60th vote it needs for a super-majority.
But American political parties are not homogenous. Legislators do not necessarily do the President's bidding, even one as popular as Barack Obama.
Until now, the threat of a filibuster sustained by the 41 Republicans in the Chamber has masked divisions among Democrats themselves. If the Republicans are reduced to 40 votes and unable to block legislation, Democrats will find themselves in an uncomfortable spotlight.
Party divisions were on display earlier this month when 26 Democratic senators from industrial and Midwest states broke with colleagues from the coasts to bar the use of the expedited budget reconciliation process to pass climate change legislation using a cap-and-trade programme.
On financial regulation, healthcare and social security, the party is deeply split between liberals anxious to push ambitious reform and centrists who favour a less radical approach. Given these divisions the President may not have 50 votes, let alone 60, with or without the support of Mr Specter and Mr Franken.
While the President may not be forced to negotiate with Republicans, the administration will still have to craft compromises between liberal and centrist Democrats. Mr Specter's change of party makes little difference. Together with Maine Republicans Olympia Snowe and Susan Collins, Mr Specter was already one of the most liberal Republicans in the Senate. In many areas, the administration could already count on his support to assemble ad hoc 60-vote majorities even as a Republican. Nothing will change.
In others, especially climate change, Mr Specter is likely to be anything but a reliable vote. Pennsylvania is a coal and industrial state. Like other senators, both Democrats and Republicans, sitting for these states, Mr Specter has been wary of any cap-and-trade programme that would not include generous grandfather rights and free permit allocations for heavy industry.
The price of securing 60 votes for cap-and-trade - a long transition period coupled with generous exclusions or free permit allocations for heavy industry and coal producers - remains unchanged.
A programme can pass this year, but the number of votes that need to be bought from within the Democratic Party means it will be heavily watered down.