Leading article: The longer the contest, the better – for Obama

The two remaining contenders for the Democratic Party's presidential nomination have held their final televised debate before Tuesday's primary in Pennsylvania. It was a grouchy affair, dominated by mutual accusations about gaffes. Hillary Clinton made the most of her opponent's snub to depressed small-town Americans as clinging to guns and religion in their bitterness. Barack Obama made just as much of Mrs Clinton's false memories of coming under sniper fire in war-torn Bosnia 12 years ago.

So far as the debate went, Mrs Clinton was judged to have won viewers' minds without conquering their all-important hearts. That pretty much sums up her biggest difficulty throughout this protracted campaign.

Even before Wednesday night's duel, however, there had been a growing clamour – mostly from the Obama camp, of course – for Mrs Clinton to leave the field. By remaining in the race, and so prolonging it, her critics say, she is jeopardising the Democrats' chances against the Republicans in November.

On the financial side, they claim that the millions of dollars each Democrat is raising and spending in order to worst the other could be more productively used to back the eventual nominee against John McCain. They also argue that the unremitting slanging match between the Democrats is handing the Republicans propaganda points that they will be able to exploit in the presidential campaign proper. And the longer the contest goes on, the more such points will be in play. Meanwhile Mr McCain has the field to himself, whether in fund-raising, campaigning or enhancing his profile abroad. His lead has lengthened accordingly.

Yet there are persuasive reasons, quite apart from her own fighting spirit, why Mrs Clinton should not feel obliged to throw in the towel quite yet – at least not before Pennsylvania Democrats have cast their votes. Mr McCain may have the financial and policy advantage at present, but US presidential campaigns fall into two distinct halves. The primary season is quite separate from the presidential race proper, which begins only after Labor Day and the party conventions. The whole tenor and pace of the campaign changes then; money spent now, like points scored, will not automatically translate into a head start come the autumn.

Then there is the matter of equity. That more Americans than ever before have had the opportunity to make their voice heard in the Democratic primaries is good for US democracy. Each presidential election year there are justified complaints that small states, such as Iowa and New Hampshire, have the first and last word in the primaries. This year has proved a refreshing exception.

And lastly, it is essential for the Democrats' prospects in November that the candidates fight on until there is a clear winner. This is the only way such a polarised party will have any hope of uniting around the nominee. What is more, if Barack Obama does eventually prevail, this long campaign will have been the perfect test of his electability as President.

Not only will he have endured everything that one of the best equipped candidates has thrown at him. He will have had ample opportunity to get the inevitable mistakes out of the way. By the autumn, when the opponent will be the old warhorse, John McCain, and the prize will not be the nomination, but the White House, a single mistake could be fatal.

Next Tuesday's primary will be crucial; it could decide the nomination in Mr Obama's favour. But if Mrs Clinton wins – and Pennsylvania is surely hers to lose – the battle will continue. No one should call this epic contest over until it is really over.

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