If the Democrats fail to take the White House this year, it will not be for the usual reason that the party chose a rubbish candidate for president. It will be because they had the rotten luck to have one too many formidable candidates. This is the horrible irony of a dragged-out, no-holds-barred, slime-slinging fight to the last delegate between Barack Obama and Hillary Clinton. The best news for John McCain from the most recent primaries is not that they formally anointed him as the Republican standard bearer. The best news for the senator from Arizona is that the Democratic Party still can’t decide who he will face in November.

The campaign for the Democratic nomination has demonstrated that neither contender is flawless. But it has also shown that both would be impressive competitors for the presidency in a year that should belong to their party. Barack Obama has displayed a remarkable capacity to inspire a genuine enthusiasm that you so rarely see in contemporary politics. Hillary Clinton offers something rather different and impresses in different ways. Her fingertip command of policy detail deserves respect. There’s a gritty resilience about the way in which she keeps cheating electoral death. This tight competition between two high-grade candidates has mesmerized the world and excited America. Hugely more voters have chosen to take part in the Democratic primaries than in the Republican contests. That is the reward for having two impressive candidates. But what was an asset is now curdling into a nightmare. They are too evenly matched. Hillary can’t flatten him; Barack can’t put her away.

Super Tuesday was a dead heat. Then he clocked up 11 straight wins. Now she is back from the dead (again) after her victories in Ohio and Texas. This guarantees that the marathon goes on at least until the Pennsylvania primary in late April. That is almost as many weeks in the future as the Iowa caucuses are in the past. And for all the speechifying, spinning and slugging ahead of them, it is highly unlikely that the remaining states can produce an unambiguous victor. It is now almost mathematically impossible for either of them to collect enough elected delegates to put them past the winning post. Fearful Democrats must contemplate the prospect of a poisonous and protracted struggle that may not even be decided before they meet for their convention at the end of August in the Pepsi Center in Denver.

There is still some chatter that this deadlock may somehow be resolved by getting the two to do a deal. There’s no deal that either candidate could find attractive if it meant submitting to the other for the sake of their party as a whole. He can claim the support of more states. She can claim the support of nearly all the big states. He appears more attractive to swing voters. She has done better in swing states. He has a narrow lead in the popular vote. She takes the lead in the popular vote if you count back in Michigan and Florida, which may rerun their contests because they were deprived of their seats at the convention for breaking party rules.

The longer this goes on, the more acrimonious it will become. The more acrimonious it is, the harder it will be for one side to reconcile themselves to a victory for the other. That the contest will become uglier is now virtually inevitable, because Hillary’s only realistic hope of conquering is by stooping. Obama still has an almost insurmountable lead over her in elected delegates. He will add to that lead if, having won Wyoming he goes on to win Mississippi tomorrow.

Attack has always been the favored modus operandi of the Clintons when their backs are against the wall. The Clintons did not just throw the kitchen sink at Obama; they bunged in the lavatory as well. For months, the Internet has been used to spread smears that Obama is a closet Islamic radical. The Clintons will also be encouraged to escalate the personal attacks on Obama because he has shown that he does not always react well under pressure. Hillary’s strategists argue that she can win them by questioning whether Obama has the experience to be president and the durability to take on the Republicans. In other words, they want to scorch a path to victory by trying to so destroy Obama’s viability as a candidate that the party’s bigwigs feel compelled to hand the nomination to her. No one ever said politics was pleasant.

This presents Obama with a nasty dilemma. If he goes down and dirty, he tramples over his central message of idealism and change. If he doesn’t fight back, he risks looking too soft to hack it.

There’s plenty of mud about the Clintons — old stuff and fresh stuff — for the Obama campaign to chuck back. Why are her tax returns unpublished? Who paid for her husband’s library? How did they accumulate so much wealth since Bill left the Oval Office? Why are the Clintons resisting the publication of their White House papers? But his dilemma was illustrated when Samantha Power, one of his senior foreign policy advisers, foolishly gave an interview in which she described Hillary as a power-crazed “monster” who would do anything to win.That’s an honest account of what a lot of people think about his rival. But it was hardly resonant with Saint Barack’s presentation of himself as a leader who rejects the “old politics” of negativity and character assassination. Ms. Power has paid for her candour with her job.

John McCain, meanwhile, blesses his good fortune. The more bitter the contest between the two Democrats, the better it is for McCain.