The considerations that should go into Mitt Romney's choice of running mate depend on whether you see his current problems as a temporary blip in the polls or a systemic problem that requires a game-changing pick. John McCain needed to throw a Hail Mary when he chose Sarah Palin in 2008. If Romney is fundamentally even with Barack Obama, he'd be better off following the Hippocratic oath: Do no harm.
In 2008, McCain needed a vice presidential candidate who could do several thing: rev up the Republican base; bring disenchanted conservatives back into the fold; peel off some women who voted for Hillary Clinton in the Democratic primaries; counteract the diversity and star power of a ticket headed by Obama. Preferably, this person needed to have a resume that would not make the selection look like pure tokenism.
From that vantage point, Palin was probably the only candidate who would have satisfied all these criteria. Her appeal to Hillary-inclined women wasn't as enduring as her popularity among conservatives, and after some bad media interviews her ability to excite the base came at the expense of outreach to swing voters. But still, it was worth a try.
Those arguing that Romney should pick Paul Ryan or Bobby Jindal, to cite just two examples, are in effect arguing that Romney needs a running mate who will help with conservatives. A Palin with a stronger resume, if you will. Someone who can fire up the base without alienating swing voters who worry about governing ability (though perhaps at the cost of some of Palin's star power). The people who want him to choose Chris Christie or David Petraeus are looking for a running mate who could potentially win over independents without alienating conservatives. Both choices would be high-risk but possibly high-reward.
Then there is the "do no harm" crowd. The people who want Romney to run with Tim Pawlenty or Rob Portman want a competent candidate with no obvious red flags for conservatives or independents, but nothing that will necessarily win over any recognizable group of voters. (Though the people championing those specific candidates hope Pawlenty will be helpful in Minnesota and, more plausibly, that Portman could help carry Ohio.) I've been in this last group until recently. I'm beginning to wonder if the campaign isn't moving in a direction where Romney will need to make a bigger splash.