In choosing the running-mate he would present to this week's Democratic national convention, Barack Obama had to decide how well, or how badly, his campaign for the presidency was going.

If all was proceeding to plan, he would have looked for a partner capable of swinging a closely contested state. This was the argument for the safe but dull Evan Bayh (Indiana) or Tim Kaine (Virginia). If the campaign was dying, on the other hand, a daring gamble to shock it back to life would have been needed: Hillary Clinton, perhaps, or even Al Gore. In naming Joe Biden, Mr Obama decided his campaign was alive but not entirely well. Characteristically, you might say, he split the difference.