Mitt Romney currently trails in the polling average by about 2 percentage points. If Romney got even half of what Kerry received or about the median vice-presidential declaration effect, then he would move into the lead in the polls. It would mark the first time all year that Romney will have lead in a majority of polls. Romney would almost certainly garner good press and perhaps some extra fundraising. It would also stop the Romneys' financials news cycle.Well, yes, Romney would likely get a bounce. After which...the bounce would go away; that's the nature of bounces. Yes, he would get good press for a few days, but then Barack Obama's campaign would start pressing him again on taxes and Bain and all, and if he continued to handle it poorly, he'd be right back where he started. If he's able to handle it well, then he should just do that now.
Sure, as Enten says, the VP bounce is also likely to produce new fundraising success. But again: so what? That bounce will happen whenever it happens, and produce whatever fundraising it will produce. There's no reason to think that the Romney campaign has a cash flow problem, so shifting a little fundraising from August to July isn't likely to matter at all.
My own sense of this is that the later, the better. Why? Because the longer Romney waits, the more relevant information will be produced. That's true even if Romney has already reached a (tentative) decision; better to leak it out in order to learn from the reaction. If they really care for whatever reason about keeping the pick a surprise (and why?), they can always leak out two or three other names between now and the final unveiling.
I still can't really see any significant advantage in doing it now.