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Mitt Romney's search for a vice presidential running mate has only just begun, but speculation among top Republicans and Romney's close allies is well under way. Here is how the list of potential running mates looks today.

Top Tier

Rob Portman - The popular senator from the important state of Ohio has twice been a cabinet level official, as well as a member of the House and Senate. He's arguably the most qualified person on the list and he helped Romney beat Rick Santorum in the Ohio primary.

Marco Rubio - His political skills are obvious as his rock-star status among conservatives. And, at a time when Republican support among Hispanics has cratered, the Florida senator is a Cuban-American who could actually deliver his acceptance speech in fluent Spanish. Rubio helps in Florida but also potentially in battleground states with big Hispanic populations like New Mexico, Arizona and Colorado.

Jeb Bush - And here is the surprise: in my top tier is the former - and still popular - governor of Florida, Jeb Bush. Romney has so much respect for Jeb Bush, he said late last year that he might not have run for president if Jeb Bush was in the race. Friends of Romney say the admiration is real. There is also obvious political appeal: Jeb Bush remains a popular figure in Florida and among Hispanics.

Second Tier

Romney's allies expect he will put a high value on executive experience in picking a running mate. That puts several Republican governors in play, including:

Chris Christie - He didn't want to run for president, but the New Jersey governor says he'd consider an offer to be vice president.

Mitch Daniels of Indiana also said no to a presidential run, but has left the door open to being a candidate for vice president.

Bob McDonnell of Virginia has strong conservative credentials, a military record and a solid approval rating in battleground Virginia. The big question for McDonnell, however, is support among women.

Also sure to get serious consideration:

Congressman Paul Ryan - He's a favorite for many conservatives and a favorite target for Democrats. They campaigned together in Wisconsin and Ryan's friends say he has quickly developed a good relationship with Romney.

Top Women Candidates

The last time Romney picked a running mate - when he ran for governor of Massachusetts - he picked a woman. There aren't many high-profile Republican women in elected office right now, but you can expect Romney to take a close look at those who are, starting with these two:

Kelly Ayotte - She's a senator from New Hampshire and former state attorney general who knows Romney well. The problem here is geographical diversity. It's a stretch to imagine Republicans picking an all-New England ticket.

Mary Fallin . The Oklahoma governor is a real long shot. She's also a former member of Congress and a lieutenant governor and is considered now to be one of the most popular governors in the country.

There's also Governor Nikki Haley , but her approval ratings have tanked in South Carolina and she has firmly denied interest in the job.

So has New Mexico Gov.  Susana Martinez .

Wildcard

Romney may also look for non-politician with national security credentials. The person who fits that bill as well as anybody: General David Petraeus. The two problems with this pick, though: first, he's currently got a job as President Obama's CIA director and; second, nobody knows if he is actually a Republican.

Others Who Will Get a Look:

John Thune  - South Dakota's three electoral votes aren't in doubt, but Thune, South Dakota's senator, was an early and enthusiastic supporter of Romney and a serious Republican player on domestic policy issues.

Jim DeMint - Mr. Tea Party, DeMint has experience, is a South Carolina senator, and would go a long way toward shoring up Romney's support on the right.

Tim Pawlenty  - His presidential campaign didn't go far, but Pawlenty, who was on John McCain's shortlist in 2008, is a successful former governor from the Midwest (Minnesota) who has become a real Romney loyalist.

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