While a decision on a running-mate could be months away, Republicans are lining up to publicly count themselves out

Florida senator Marco Rubio and Virginia governor Bob McDonnell have participated in the long-established ritual of ruling themselves out as running-mates for Mitt Romney, who on Wednesday enjoyed his first full day as undisputed Republican presidential candidate.

Rubio and McDonnell join a growing queue of Republicans who insist they do not want to be the Republican vice-presidential candidate – but they have to say that, concerned they would end up looking foolish if they express an interest and Romney fails to pick them.

McDonnell, interviewed on CBS on Wednesday, said he was happy being governor of Virginia. "I got the job held by Thomas Jefferson and Patrick Henry – governor of Virginia. Doesn't get any better than that," McDonnell said.

With Rick Santorum's departure from the race on Tuesday, speculation began in earnest over the last big vacancy of the 2012 campaign to be filled, even though Romney is unlikely to announce his choice for months. Learning from the mistakes of John McCain's team in 2008, Romney's team will ensure their choice, unlike Sarah Palin, will be thoroughly vetted.

Santorum has expressed interest in the post but his harsh criticism of Romney would be exploited by Democrats, repeatedly replayed in ads."I think Santorum probably is not going to be in line for the VP spot because it's just been too nasty and contentious," Bradley Blakeman, a politics professor at Georgetown University, told Newsmax. "I think that he certainly can serve in a Romney administration if they find the right place for him where he can be helpful."

Rubio is repeatedly cited as among the frontrunners because he is a Latino, a demographic group that Romney and other Republicans have alienated, and because he represents Florida, a swing state that could determine the outcome of the White House race. "I'm not going to be the vice-president," Rubio said, a line he has been repeating since last summer.

But if offered the post, it would be hard to resist a platform from which to launch a bid of his own for the presidency in 2016 or 2020.

Susana Martinez, the governor of New Mexico, is another prospective vice-presidential candidate, with the advantage of not only being from a Latino background but a woman, a demographic group with which Romney struggles compared with Obama. Martinez, like Rubio, has ruled out the job. "It's humbling but I'm not interested," she told the Associated Press.

Much of the speculation focuses on issues such as gender, geography or ethnic background, working on the assumption that these will help bring in voters on the margin. But recent political history shows that presidential candidates have tended to buck this received wisdom, spectacularly so in the case of McCain in picking a running-mate from Alaska, which has almost no impact on the presidential race, and a woman who alienated women in droves.

Conservative commentator George Will, writing in the Washington Post, noted that since the second world war, 10 presidential candidates have failed to win the home state of their vice-presidential running mates. He argued instead in favour of two candidates experienced in budgetary issues and adminstration, Louisiana governor Bobby Jindal or Paul Ryan, chairman of the House budget committee.

"So Romney's running-mate should have intellectual firepower, born of immersion in policy complexities, sufficient to refute Obama's meretricious claims and derelictions of duty. Here are two excellent choices," Will writes.Both Jindal and Ryan have ruled out being Romney's running-mate, as have New Jersey governor Chris Christie, former Minnesota governor Tim Pawlenty, South Carolina governor Nikki Haley, and Ohio senator Rob Portman.

One of the few Republicans to publicly express an interest in the job is Herman Cain. Part of the reason may be that he, unlike others, have opted to say he would consider it is because he is a long shot, having been forced out of the Republican presidential race late last year amid allegations of sexual harassment.

Asked by Fox how he would react if approached by Romney's campaign team about the job, he said: "I would say 'Let's talk'. It's not a slam-dunk."

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