The vice presidential speculation is only just beginning. Most activists and pundits agree that Sen. Marco Rubio (R-Fla.), New Jersey Gov. Chris Christie and Virginia Gov. Bob McDonnell are the front-runners for the gig. But who has taken himself out of contention?

The leader in that department is certainly Rick Santorum. It isn’t unusual for the nominee to take the runner-up (even one who has lobbed grenades at the eventual winner), but in this case Santorum’s negatives far outweigh his positives. He can’t claim to be a draw with blue-collar voters in the Rust Belt or Catholics. His spending and earmark record isn’t going to soothe the jangled nerves of small-government conservatives. And maybe most of all, he has proven to be both controversial and undisciplined, two big no-nos in Romneyland.

Also in the category of “don’t sit by the phone” is Tim Pawlenty. He wasn’t a successful surrogate, even in his own state. In fact it’s far from clear whether he could carry his own state in November. Moreover, he will neither wow the base nor make inroads with any constituency (e.g., Hispanics). He’s just not a very good national campaigner.

Then there is Indiana Gov. Mitch Daniels. For a long time he was at the top of the many Republicans’ short list for president. But he has yet to endorse Mitt Romney or even offer well-timed, helpful comments (as Rubio has served up). In addition, the idea has taken hold that Romney’s big problem is that he doesn’t generate excitement and isn’t a compelling figure to fire up the base. Well, if he needs some spontaneity and excitement, Daniels is not the go-to guy.

You can add Ohio Gov. John Kasich to the “not going to happen” list. In his state’s hotly contested primary, he didn’t endorse Romney. And if he had, it’s not clear how much he would have helped. His approval rating has been in the 30s, although he did hit 40 percent in one poll. Moreover, Sen. Rob Portman (R-Ohio) has become a favorite Buckeye State surrogate of the Romney camp. If the Romney team feels like it needs help in Ohio, Portman (who’s approval rating is right side up) is the more logical pick.

Finally, any lightly experienced pol who would be seen as an obvious attempt to curry favor with one or more voting blocs (e.g., New Mexico Gov. Susana Martinez, Nevada Gov. Brian Sandoval, South Carolina Gov Nikki Haley) would have a very steep hill to climb to get into contention. Fairly or not, Sarah Palin stands as the uber-example of what happens when a solid conservative is prematurely thrust into the national limelight. VPs don’t win the presidency, but they can do great damage. And if the headline is “Is X Romney’s Sarah Palin?” it will be clear that Romney has made a serious misstep.

For Romney the short list is unlikely to include inexperienced, unfocused, untested and less than stellar conservatives. If he can find a running mate who can pump up the base and maybe extend his appeal, all the better. But the first rule is: Do no harm. And given that Romney is a careful, meticulously organized person, you can probably amend that to: Don’t even risk doing harm.