Leading article: A victory that only extends the agony of the Democrats

When the polling stations opened in Pennsylvania on Tuesday, many US Democrats hoped, and others feared, that the party's protracted primary contest would be over by midnight. With a lacklustre victory – or none at all – Hillary Clinton would have graciously conceded, acknowledging that, if there was to be another Clinton in the White House, it would not be in 2009. Barack Obama would by now be considering his strategy against John McCain, the chance to become America's first black president tantalisingly within his grasp.

The real-life script unfolded differently. Mrs Clinton prevailed with the double-digit advantage she needed. As in New Hampshire and then in Texas, the result defied forecasts that her rival was snapping at her heels. Mr Obama will have to be patient a little longer; patient and extremely careful. Even if he wins convincingly in Indiana on 6 May, he will still not be home and dry. The greater the pressure Mrs Clinton senses, the more impressively she fights; becoming the underdog was the making of her candidacy. With every day that passes, she is proving – as her husband proved in his time – that down, for a Clinton, does not mean out.

It would be wrong to underestimate what Mr Obama has achieved. Inspirational, mostly unflappable and remarkably sure-footed for a relatively inexperienced senator, he snatched the advantage early from a former first lady with flush with cash and the country's most formidable politician in her entourage. He has shown himself more adept than she at attracting the fickle youth vote, mobilising black Americans and maximising the opportunities afforded by the internet. He has kept the money flowing in, even as Mrs Clinton's coffers were depleted.

The risk for him, however, is that the next stage of this enthralling battle is likely to be about more than numbers, personal magnetism, or even staying power. Within hours of their candidate's victory in Pennsylvania, the Clinton team was arguing that she alone had demonstrated the power to attract centrist Republicans – the swing voters who could decide this election, as they have done previously open elections. And the slightly awkward fact for Barack Obama is that his clearest victories have by and large been in small states which are predominantly Republican. In those big states where Democratic voters are in the majority or centrist Republicans could perhaps be won over, it is Mrs Clinton who came out ahead.

The argument that Hillary Clinton is more electable against John McCain in November is likely to be a central pitch of her campaign from now on – with the white working class of Pennsylvania adduced as evidence. Mr Obama can parry this by citing his advantage in the popular vote which, natural justice would suggest, should make the nomination his. It could nonetheless be a difficult call.

Mr Obama has much to gain in the crucial weeks to come, but also much to lose. He must avoid mistakes; he must keep his cool. He cannot afford to show any peevishness or arrogance. Above all, he must demonstrate by the power of oratory and argument that he would make not only the stronger nominee against John McCain, but also the better US President.

To do this, he will need to be more precise about his policies and readier to exploit the unusual breadth of his personal experience. The way in which he dealt with the row about his pastor, Reverend Jeremiah Wright, showed that he has the necessary range and depth. After Pennsylvania, though, he has no choice but to broaden his appeal further. Mrs Clinton has demonstrated once again how difficult she will be to beat.

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