There is one more reason why I think it would be a bad idea for Obama to exchange Joe Biden for Hillary Clinton as his VP choice. While Clinton and Biden are different as politicians, and it would certainly be possible to argue that Clinton is more competent than Biden, the most obvious difference between the two of them is of course that Clinton is a woman and Biden is a man. If Obama suddenly exchanged Biden for Clinton, he would effectively be saying to a majority of the American voters that he now realizes that a female VP is better than a male one because a VP's gender matters more than his or her beliefs, competence or qualifications. That, however, would shift everybody's attention even more from the political issues themselves - the financial crisis, the energy question, America's foreign policy etc. - to the the candidates' own persons. The fact that Barack Obama is black is probably more of a drawback than an advantage for him, and precisely because of that he should try to focus on the issues and not on the race or gender of the politicians.
Ann
