When the history of this absorbing US presidential election comes to be written, the past week may be seen as pivotal. For the Republicans, John McCain clinched the nomination and yesterday won the endorsement of Mitt Romney, the man initially thought to be his chief rival. But it is on the Democratic side that the most dramatic shift has taken place. Hillary Clinton can no longer claim to be the front-runner, and for the first time serious doubt is being cast on her ability to win the nomination.
In part, the shift is about delegate numbers – but it is not only about numbers. It is also about momentum and mood. The role of "super-delegates" at the Democratic convention means that Mrs Clinton could prevail there even if the numbers are close. That assumes, however, that she retains the support of the party establishment and her machine is capable of going into overdrive. Neither is now at all certain.
Barack Obama won this week's so-called Potomac primaries by even more impressive margins than forecast. And while the Obama camp rejoiced, Mrs Clinton was trying to retune her supposedly invincible machine. She is said to be having difficulty raising money – and she will need enormous sums if she has to fight right through to the convention. For Mr Obama, it is not only that his unexpectedly strong showing has bred fundraising success, but it is also that his grass-roots fundraising operations have shown themselves to be flexible and responsive. As the campaign powers on, his many small donors are proving more reliable than the – perhaps now wavering – big corporations.
And the mood in the two camps could hardly be more different. As Mr Obama allows himself ever more lofty flights of rhetoric, Mrs Clinton is casting around for a new message to stall Mr Obama's rousing promise of "change". As he wins more converts by the day – including Republicans unhappy with their own party's choice – Mrs Clinton suffers only defections. Somehow she will have to reverse this tide if her campaign is to retain its credibility.
With the Republican nomination decided, however, many of the Democratic primary voters' calculations have altered. They are no longer weighing the competing merits of their two candidates as potential presidents, but considering which has the better chance of beating Mr McCain. And whereas the presidential advantage probably lies with Mrs Clinton, the electoral advantage belongs to Mr Obama. The depth of hatred of Mrs Clinton among right-wing Republicans would unite the party around Mr McCain to "stop Hillary". Against Mr Obama, Mr McCain could count on no such solidarity. If Democrats want to win back the White House – which they surely do – it is Mr Obama who has the better chance of doing so.
The race is by no means over for Mrs Clinton. Having lost the clutch of primaries on Tuesday, she won the caucuses in the small state of New Mexico two days later, suggesting that her support among Hispanic voters has held up. This could be crucial on 4 March when Texas and Ohio hold their primaries. And Mr Obama is still in many respects an unknown quantity. Unpleasant surprises cannot be excluded, if primary voters allow themselves to be carried away by his inspirational gifts without considering also soundness and substance.
Increasingly, however, it looks as though Barack Obama's chief distinction resides in being the very opposite of the tongue-tied and polarising ideologue who currently occupies the White House, and presenting himself as a man of the future, not the past. That he may have victory within his grasp is the realisation that has crystallised at the end of this decisive week.