Mitt Romney: Here are his most likely choices for vice president

With Rick Santorum finally out of the Republican nomination race and Newt Gingrich's efforts dwindling, the focus now turns to the campaign's next phase: Who will be Mitt Romney's running mate?

Leading the list of potential frontrunners is freshman Florida Sen. Marco Rubio.

"He has the appeal. He's young and the conservatives like him," said Stuart Rothenberg, political analyst. "The question is, does he have the gravitas?"

Geoff Skelley, media relations coordinator for the University of Virginia Center for Politics and political analysis website Sabato's Crystal Ball, thinks he does.

"He's the sexiest pick of the choices," Skelley said. "The Republicans need the Hispanic vote, and he's from Florida, an important swing state."

Still, Rothenberg said geography isn't the most important factor.

"Republicans are going to win the South, if they don't, it's not going to be because Romney picks a running mate from Jersey," he said.

Indeed, that Jersey running mate may be a possibility, as Gov. Chris Christie is second to Marco on Intrade with a 10.7% rating. He has wavered back and forth on whether he would jump into the 2012 race in one capacity or another, but most recently said he'd consider a VP offer from Romney.

"It'd be really interesting, potentially terrific," Rothenberg said. "He's not a scripted, blow-dry politician."

Kyle Kondik, a contributor to Sabato's Crystal Ball, disagrees with Rothenberg, saying Christie's "boisterous presence" could overshadow Romney.

Still, Rothenberg said not to rule out Christie.

"Just because someone says they're not interested doesn't mean they wouldn't take it if they were nominated," he said.

He added that dismissing speculation is a "standard response" at this stage.

Rounding out the list of highly discussed VP options are Virginia Gov. Bob McDonnell, Wisconsin Rep. Paul Ryan and Ohio Sen. Rob Portman.

Portman, a seasoned politician, has a strong background in economics – he was the director of the Office of Management and Budget – which could add to Romney's economy-first platform.

"The question is, 'is Portman too safe a choice?'" Skelley said, adding that Portman wouldn't bring any diversity to the ticket.

The economy itself, too, will play a major role in the election.

"There will be seven months of economic news to digest. There could be all kinds of things that could happen. You can't even guess what," Skelley said.

Costas Panagopoulos, associate professor of Political Science at Fordham University, said said if there are "dramatic signs of improvement," it could become an "uphill battle" for Romney, but if there is a downturn or "mixed signals," his chances "are better."

"Better," but no "better than 50/50," he concluded.

But regardless of whom Romney ultimately chooses, in the end, it all comes down to him, Rothenberg said.

"Romney is going to win or lose this election," he said. "Not the vice president."