Rick Santorum said last week he would be a vice presidential candidate to Mitt Romney, if he couldn't be the nominee himself.

The Romney campaign isn't interested, a top adviser said Monday.

"You want somebody who adds to the ticket," Romney surrogate and former New Hampshire Gov. John Sununu told National Review Online. "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 said Santorum has shown "softness under pressure" that he believes would be a problem as a vice presidential nominee. Sununu added that the vice presidential pick -- who he emphasized has not been chosen yet -- will be someone who differs from Romney, including being from a different region.

"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," he said.

Two names being floated as potential VP nominees, particularly after they endorsed Romney last week, are Sen. Marco Rubio (R-Fla.) and Rep. Paul Ryan (R-Wis.), each of whom come from different regions and can tout strong conservative credentials on the economy.

Romney joked on "The Tonight Show with Jay Leno" last week that he associates Santorum with the words "press secretary."

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