If Biden believes Obama will win, why continue Senate run?

That's the part of the whole Biden story that doesn't make sense to me. If Senator Biden truly believes that the Obama/Biden ticket will win, then why would he continue to campaign for his senate seat? We seem to have accepted that our elected officials will campaign for other offices while supposedly representing us, since Obama, McCain and Biden are all currently Senators, but isn't campaigning for both the vice presidency and the senate at the same time just a bit much?

It sends the message that Biden does not have faith in the presidential campaign, else he would step down to allow someone else the chance to run for that seat. If Obama wins, the voters of Delaware will not have a voice in who will represent them, that decision would be made by the governor of Delaware and it's being suggested that Biden wants one of his sons to have that spot.