The real reason why an Obama-Biden ticket is a vulnerable one is that it plays into one of the few strengths that Republicans and John McCain have in the 2008 election. A weak economy, a surge in Democratic voter registration, an unpopular Republican president, a sour public mood, a desire for political change -- all those favor Mr. Obama and Democrats. But what Mr. Obama has done is create an all-liberal ticket -- a very, very liberal one, at that -- in a nation whose electorate is still center-right. The political mood may be a bit more centrist today than...