John McCain isn't offering any names yet for a possible vice presidential pick, but the presumptive Republican presidential nominee said Sunday the candidate doesn't have to be a foreign policy guru.

McCain, whose own credentials lean toward foreign relations, said his preference would be someone with a lot of leadership experience who shares his values.

I think that that person, the first and really major overwhelming priority is the person shares my principles, my values, my priorities ... and could be -- immediately take my place. That's, I think, the overriding criteria," McCain told "FOX News Sunday" in an interview taped Friday.

"Frankly, the process that we're in, it's so early in the process, and honestly, it's the sole criteria I'm looking at is not that. It is who can best take my place and carry on the agenda and the vision that I have outlined and will continue to outline during this campaign," McCain said.

As the Arizona senator scours through a list of what he previously estimated was about 20 potential running mates, fans of former McCain rival Mike Huckabee are issuing an open letter to McCain telling him that if he chooses Mitt Romney, another former rival, he will lose the conservative vote.

"For us the bottom line is this: The unvarnished facts of Mitt Romney's record as governor of Massachusetts make him utterly unacceptable as a vice presidential choice," reads a newspaper ad signed by 26 conservative leaders who have formed the Government is Not God political action committee. Among the ad's sponsors are Troy Newman of Operation Rescue, Sandy Rios of Culture Campaign and Paul Weyrich of the Free Congress Foundation.

A petition on the NoMittVP Web site that accompanies the ad says that the sponsors want McCain to choose someone with a consistent record of supporting the sanctity of human life as well as a supporter of a constitutional amendment defining marriage as between one man and one woman.

"Willard Mitt Romney is a dealbreaker," reads the ad scheduled to appear in newspapers in towns where McCain is campaigning. Most of the ad's signatories are Huckabee supporters, but Weyrich formerly backed Romney, issuing a statement in November saying Romney shares his conservative values.

Romney's press secretary told FOX News that the claims made in the ad are not true, but the group's efforts are wasted anyway.

"It strikes me as very silly. First, the claims that are being made are false and have been thoroughly discredited, and second, Governor Romney does not expect to join the ticket as vice president," said press secretary Eric Fehrnstrom. "Governor Romney's focus is on helping Republicans get elected, and he believes the way to win in November is by unifying against the Democrats."

Asked whether he was part of the ad campaign, Huckabee, the former Arkansas governor and a Baptist minister, said he didn't help coordinate the ad and didn't have a position on it.

"I had nothing to do with the content or placement of the ad," Huckabee told the newspaper. "I know somebody had e-mailed it to me beforehand." Huckabee added that he doubted McCain would pick Romney anyway, "given their deep philosophical differences."

Romney has previously said that he'd be honored to be on McCain's ticket, and some of his supporters are apparently lashing out at the ad's sponsors. The NoMittVP.com Web site has posted an advisory saying it is not a blog site so Romney supporters shouldn't sign the petition just so they can make "vulgar comments."

"If you support Romney you should not be signing a petition against him in order to make pro--Romney statements. That is flat out dishonest," reads the site.

Click here to read the NoMittVP Web site and ad.

Click here to read The Washington Times article.