Here's why: the backlash to Clinton has been because of previous trash talk about Obama. How can she consider a running mate who lacks even her manly bona fides -- and even worse, she says, those of evil Republican John McCain?
But, if Obama holds Clinton to her offer to consider teaming up it makes it virtually impossible for her to go negative.
The latest tortured explanation as to how Obama might after all be ready to be commander in chief on day one (as he'd need to be as VP, a heartbeat away from the presidency) came from Clinton spokesman Howard Wolfson on Morning Joe: there's a lot of time between now and the nominating convention in August. Joe Scarborough later wondered out loud if that meant they'd be sending Obama to Fort Dix for a few months.
Obama's instinct to react sarcastically for the crowd's benefit was the correct one. But he also has to be just as careful as she not to offend the nearly half of Democrats who favor the other candidate.
Obama needs to to quickly pivot and get a buzz going that the Brother/Sister ticket (In that order. Just sounds better) would be unbeatable.