Note: I'm not married to the idea of a Clinton Vice-Presidency and the author of the piece quoted below was not a Clinton supporter in the primaries.11 Reasons Hillary Should Be The VP Pick.
Her political future will be dependent on your success; therefore she has every interest in promoting your agenda. The alternative (without her on the ticket) is much more problematical. She will be a huge force in the Senate with her own base and agenda. Would you rather have her out of the tent on the Hill promoting her own ideas or in your tent promoting yours? Not even a close call.
A few other benefits of Clinton on the ticket; no one will be a more affective attack dog against McCain and the Right than Hillary. She can take the heat and defend you (something you are increasingly forced to do yourself). Every attack on McCain by Clinton will get wide coverage. No one has had more experience than Hillary on taking the Right to the wood shed and beating them to a pulp. She becomes the lightening rod, you go back to change and hope. Can any other VP choice do that? Not even close.
You will sit atop the most unified Democratic Convention since the advent of television. The picture of you and Hillary Clinton together is a nightmare for Republicans. The GOP operatives are yearning for you to pick anyone besides Clinton. You scare them, she scares them, together you are their worst nightmare.
Finally, and maybe most important of all, McCain and the Right have only one hope of defeating you Senator, and that is to make you too naïve and risky to be Commander in Chief. Your vice presidential choice must reek of experience. The argument against Clinton is she represents the past and hurts your change message. Sure the voters want change from what hasn't worked, but they embrace past success. Most voters recall the Clinton years (from a policy perspective at least) as successful. A message of change combined with a record of past success is a more comforting message than change alone. Can another VP candidate provide that level of comfort? Not even close.
http://www.realclearpolitics.com/articles/2008/06/its_not_even_close_obama_shoul.html