There was a poll last year after Trump was "in the running" that asked if one would be more or less likely to vote for a candidate if they were endorsed by Trump. Something like 60% said they'd be less likely to vote for that person if Trump endorsed them.
Him coming back into the spotlight could probably help Obama way more than attacks on Bain could.
What makes it worse is that Romney's campaign was promoting a dinner with him and Trump for donors.