Nate Silver has a very comprehensive post looking at the benefits and risks of a Romney VP choice. The bottom line is pretty easy to state: he potentially helps with business-oriented centrists in the Northeast and Midwest, and clearly helps with Mormons in the West, but he could be anathema to Evangelical Protestants, particularly in the South. A Romney choice, in other words, would be a mirror-image of a Huckabee choice in terms of costs and benefits: Huckabee would excite the Evangelical Protestant base, and potentially lure other blue-collar voters with socially conservative views, but would cause Northern business-oriented centrists to take a hard look at Obama as the safer choice, and would absolutely repel Mormons in the West (who remember the anti-Mormon messaging of the Huckabee campaign all too well).
The bad news for McCain is that he doesn’t have any really great choices for VP. The two most obvious choices, as noted above, are each very problematic. Pawlenty is safe, but isn’t going to excite anyone; ditto Thune; ditto Daniels; Ridge would actually produce negative excitement; ditto Engler; Palin is too young and untried (though she’s the most plausible “exciting” choice – certainly more so than Jindal); Fiorina is not a serious choice; neither is Lieberman; Rice isn’t interested and is too closely identified with the Bush Administration; I don’t see what Cantor or Portman bring to the table; and so it goes. The good news, though, is that his choice doesn’t matter so much. McCain is already well (and positively) defined for this election. He’s the old soldier who isn’t too great with policy details but who loves the American people and always puts them first. If he can keep the Obama campaign from tarnishing that veneer, and picks someone as his VP who is modestly helpful as attack dog and potential successor, he’ll be in as good shape as he can be given how badly the fundamentals cut against him and his party.
The good news for Obama is that he has lots of good choices for VP. Now that Webb is out, the three choices that seem most interesting to me are Schweitzer, Rendell and Bill Bradley. Schweitzer is another new-generation Democrat; he puts Montana into play; he’s got a mix of policy expertise that nicely complements Obama’s and that reinforces his campaign themes – he’s a good “change” choice. Rendell would be a great attack dog for going after McCain (he’s another guy with a “straight talk” reputation), and he’d bolster Obama among downscale white Catholics who he’ll need to do better with than he has to win Ohio; if Obama can consolidate Democratic support, he probably wins, and Rendell would be a good choice for that purpose. And Bradley would basically lend gravitas and depth to Obama’s existing positioning as a pragmatic liberal, without overshadowing the nominee (Bradley is famous, popular and well-regarded, but nobody considers him a political powerhouse or especially inspirational); he’d be a reassuring presence to both independent and Democratic voters mainly concerned about the fact that Obama is an unknown quantity. I’m least enthusiastic about Kaine, just because I think he’s almost a pure geographic play without much else to offer (I don’t think Obama has an obviously strong Southern choice now that Webb is out), and I have concerns about whether Sebelius is ready for prime time; and Bayh, while he makes so much sense on paper, is so profoundly boring a choice that I really hope he’s not the one; but Biden is reasonable, Reed is reasonable – Clinton would be an interesting choice if it weren’t for her husband – point is: he’s got a lot of reasonable choices. But the bad news is: compared to McCain’s choice, the stakes for his are a whole lot higher. The public has much less of a defined impression of Obama than of McCain, and so Obama’s VP choice will do more to shape the narrative going forward than McCain’s will.