Two weeks ago in this space I expressed the naive hope that a US presidential contest between John McCain and Barack Obama might be a cut above ordinary politics. Neither man, to put it mildly, is the conventional type. Both are men of principle, with strong convictions – but with a pragmatic streak as well, open-minded, committed to bipartisan co-operation and running against business as usual. With luck, I said, they would treat each other with respect and steer clear of ad hominem smearing. For once there might be an election about the issues.

Perhaps I misspoke. Mr Obama, increasingly certain of his nomination as the Democratic candidate despite Hillary Clinton’s refusal to yield, has begun turning his attention to Mr McCain. His principal line of attack is that the Republican nominee stands for “four more years of George Bush”. Mr McCain, meanwhile, has fastened on his rival’s avowed willingness to meet rogue leaders such as Iran’s Mahmoud Ahmadi-Nejad “without preconditions” and charged him with a taste for appeasement.