The online betting site lists 20 vice presidential nominee contenders ranging from feasible picks like Rubio to more outside the box options such as Herman Cain, who has a 15,000 to 1 shot of being selected, according to sportsbook.com.
At 9 to 5 odds, Rubio has the best chances of being picked as Romney's vice presidential nominee, sportsbook.com predicts.
Rubio campaigned with Romney in Pennsylvania late last month and is considered to be one of the frontrunners for the VP slot.
Rubio hails from a key battleground state -- Florida -- and Rubio being Latino may help Romney pick up support among Hispanics.
Close behind Rubio in the veepstakes is Rob Portman, the immensely popular Ohio senator, who sportsbook.com gives 11 to 5 odds of being Romney's vice presidential pick.
National Journal reporter Major Garrett told CBS News he believes Portman will be Romney's guy.
Everything I know about Rob Portman, everything I know about Mitt Romney and the way he makes decisions tells me it's going to be Rob Portman because it emphasizes the central message of the campaign... he's not exciting, but he's well vetted, Garrett said.
Virginia Governor Bob McDonnell has the third highest odds of being the vice presidential nominee, with sportsbook.com giving him an 8 to 1 shot of being No. 2 on the Romney ticket.
House Majority Leader Eric Cantor, McDonnell's fellow Virginian, is pushing him for the spot.
He is somebody who has steered our state through tough times. I think if you look at our unemployment rate, our growth rate, we fare a lot better than most, Cantor said last month on CNBC's The Kudlow Report. He's a great guy. I think he'd be a great vice president.
Popular New Jersey Gov. Chris Christie, whom many Republicans wanted to get into the Republican primary race, is also in the mix for the VP slot.
Christie said it wasn't his time to run for president but is leaving his options open when it comes to possibly being vice president.
I really have no interest in being vice president. But if Governor Romney called and asked me to sit down and talk to him about it, I'd listen, Christie told the Newark Star-Ledger Monday. Because I think you owe the nominee of your party that level of respect and who knows what he's going to say. He might be able to convince me, he's a convincing guy. But I really love this job, I really want to stay in this job.
Also on sportsbook.com's list are U.S. Rep. Paul Ryan (R-Wisc.), Indiana Gov. Mitch Daniels, U.S. Sen. Kelly Ayotte (R-N.H.), New Mexico Gov. Susana Martinez, former Minnesota Gov. Tim Pawlenty, Louisiana Gov. Bobby Jindal, former U.S. Secretary of State Condoleeza Rice, recently withdrawn GOP presidential candidate Rick Santorum, U.S. Sen. Rand Paul (R-Ky.), South Carolina Gov. Nikki Haley, former Florida Gov. Jeb Bush, former Arkansas Gov. Mike Huckabee, U.S. Rep. Michele Bachmann (R-Minn.), former Alaska Gov. Sarah Palin, former House Speaker Newt Gingrich and withdrawn presidential candidate Herman Cain.
Of those contenders, Martinez said she is not interested in being on the ticket and some on the list are longshots such as Cain.
Here is a list of the 20 possible contenders and their odds, according to sportsbook.com:
Marco Rubio: 9 to 5
Rob Portman: 11 to 5
Bob McDonnell: 8 to 1
Chris Christie: 15 to 1
Paul Ryan: 15 to 1
Mitch Daniels: 15 to 1
Kelly Ayotte: 20 to 1
Susana Martinez: 25 to 1
Tim Pawlenty: 25 to 1
Bobby Jindal: 25 to 1
Condoleeza Rice: 30 to 1
Rick Santorum: 30 to 1
Rand Paul: 30 to 1
Nikki Haley: 30 to 1
Jeb Bush: 40 to 1
Mike Huckabee: 50 to 1
Michele Bachmann: 100 to 1
Sarah Palin: 100 to 1
Newt Gingrich: 100 to 1
Herman Cain: 150 to 1