Former New Hampshire governor John H. Sununu tells National Review Online that “credibility” will be a critical factor in the vice-presidential selection process, should Mitt Romney win the Republican nomination, and that Rick Santorum, Romney’s rival, will probably not be a contender for the coveted number-two slot.

“You want somebody who adds to the ticket,” Sununu says. “You also want somebody who has the right credentials and is credible — somebody who the public agrees could be president. That filter gives you seven to ten people to choose from, and then you choose who among them gives you the best political impact.”

Sununu, a Romney confidant, doesn’t expect Santorum to make that list. “He has shown, emotionally, softness under pressure. If anyone asked me personally, I would suggest that it’s a problem,” he says.

Sununu also does not expect a Republican from New England to be tapped. “If it’s Romney, you will not have a vice-presidential nominee from the northeast,” he says. “Geography is a factor. You don’t want to be accused of being so parochial that you can’t even look outside of your own backyard.”

But Sununu cautions that Romney’s advisers are not actively mulling the veep field. “It’s a little early,” he says. “Right now, you can evaluate the credentials of [possible picks], but you can’t do the politics. The politics of July and August will be different than the politics of April and May, and the politics of July and August will be closer to the politics of November.”