Rupert Cornwell: Murphy's Law has put its curse on Romney

It's tough these days being Mitt Romney. This wasn't a bad week for him – nowhere near as bad as the one before, which kicked off with the now infamous "47 per cent of Americans feel themselves victims". The Republican challenger has been out on the campaign trail more. He made a decent foreign policy speech in New York too, and even managed a decent joke (about the benefits of being introduced by Bill Clinton).

But Murphy's Law is at work: anything that can go wrong for Mr Romney will – the latest example being Americans' growing acceptance of President Obama's healthcare reform. Mr Romney (who as Governor of Massachusetts pushed through a similar reform at state level) is now having to say that, if elected, he will preserve sizeable parts of the 2010 law. Expect his feet to be held to the fire on this issue in the first presidential debate on 3 October.

The debates are supposed to be Mr Romney's last chance of a comeback. But history shows they very rarely change things. In terms of debating skills the two candidates are evenly matched, and at this late stage most minds are made up. Only a mega-howler by Mr Obama will make a difference on Wednesday. Far more crucially, the swing states seem to have turned against the challenger. In Ohio, the President leads by up to 10 points, and in Virginia, Iowa and New Hampshire his lead is now beyond the margin of error.

Worst of all for Mr Romney, an air of inevitability is settling. In US politics, foregone conclusions usually turn out that way.