Republicans should be so lucky -- for several reasons.
First, there goes the "change" rationale for Barack's candidacy. If he's simply going to bring Hillary right into The White House anyway, how does he justify having run against her -- or, given the closeness of the vote and the experience differential between them, being at the top of the ticket rather than #2? What's more, see how well the choice plays out with his key constituency of young people -- who may feel duped that they actually believed the hype about him being different.
Second, the calculus involves an assumption that may well be unwarranted -- namely, that Hillary can actually deliver the 18 million voters who supported her to the Democrats. At the outset, one had better be sure they were really voting for her, and not against her arugula-eating, elitist primary opponent. Then, they'd better hope that Hillary's supporters don't figure that it makes sense to sit this one out, now that their candidate's the guaranteed frontrunner for 2012, rather than working hard to install her as second banana until 2016, when she's almost 70.
Third, a Hillary pick would reveal several things about Barack's M.O., none of them flattering. It's obvious that he wouldn't pick Hillary and bring along all the Bill Clinton dysfunction, the vetting questions, and the upstaging unless he felt he had to. So then it becomes a matter of whether he overreacts, panicking at the recent disturbing poll numbers -- and enough so to be willing to mortgage his ability to act independently as President (without Bill and Hillary baggage) just for the sake of winning. That certainly wouldn't reveal anything too flattering about his "crisis" decision-making.
Or could he be naive enough -- or arrogant enough -- to believe that he can do what no other Democrat has been able to: That is, to shunt Bill and Hillary aside?