WASHINGTON (MarketWatch) — At some point in the next three weeks, Mitt Romney will lift the veil on his running mate, to the delight — or disappointment — of his fellow Republicans.

A key consideration for Romney is readiness to assume the presidency if succession ever becomes an issue. But Romney has many other factors to consider when making his vice-presidential pick, like an ability to raise funds, a good personal fit, and knowledge of the economic problems facing the country, to name a few.

Various lists and shortlists abound in the political universe as the clock ticks down to the Republican convention in Tampa later this month. MarketWatch asked experts about the pros and cons of three of the people frequently mentioned on those lists: former Minnesota Gov. Tim Pawlenty, Ohio Sen. Rob Portman and Florida Sen. Marco Rubio.

To be sure, there are other rumored contenders for the slot below Romney on the ticket, including New Jersey Gov. Chris Christie, Louisiana Gov. Bobby Jindal, Virginia Gov. Bob McDonnell, former Secretary of State Condoleezza Rice and House Budget Committee Chairman Paul Ryan.

And, moreover, political scientists say that Americans don’t vote for vice presidents. But here are some of the ways that three of the top contenders could impact Romney.

Former Massachusetts Gov. Mitt Romney (from left), with U.S. Rep. Ron Paul and former Minnesota Gov. Tim Pawlenty at a New Hampshire debate in June 2011. Reuters

Tim Pawlenty: The former Minnesota governor, who abandoned his own 2012 presidential bid, is almost universally seen by political observers as a “safe” choice for Romney. Pawlenty’s advantages, according to vice-presidency expert Joel Goldstein, include having been vetted when he launched his White House run; his Midwestern roots; his evangelical Christianity; and an ability to offer a contrast with the famously wealthy Romney.

“Pawlenty has the narrative of being the son of a truck driver,” says Goldstein, a professor at the Saint Louis University School of Law. If Romney wants to tap someone with blue collar roots, in other words, Pawlenty may be the man.

The minuses of picking Pawlenty? One is a lack of national-security credentials, says Goldstein. Moreover, his unsuccessful run for the presidency may raise questions about the ticket’s dearth of broad appeal to voters. Plus, says Emory University professor Alan Abramowitz, even with Pawlenty at his side, Romney’s chances of winning the North Star State are very low.

“I don’t see Romney carrying Minnesota,” says Abramowitz. (President Barack Obama leads Romney in Minnesota by more than 10 points in the latest average of polls from RealClearPolitics.)

Portman joins Romney in phoning potential South Carolina primary voters in January. Reuters

Rob Portman: Portman, a first-term Ohio senator who ran President George W. Bush’s Office of Management and Budget, also frequently has the “safe” label attached to him — with a few exceptions. True, says Goldstein, the swing-state native is perceived to be a “very able guy,” having served Bush at OMB and as the 43rd president’s trade representative. But those same ties to the former president could be damaging to Romney. It’s probably not lost on the former Massachusetts governor that Bush was very unpopular when he left office. Neither Bush nor former Vice President Dick Cheney are speaking in Tampa.

On the plus side, Portman has been extensively vetted — having run for Senate and been confirmed for two administration posts — so there’s almost no chance of unseen skeletons emerging from his closet. A question mark is whether he would help with Ohio, a must-have for Romney. Neither Abramowitz nor Goldstein believe Portman would make the Buckeye State a slam dunk for Romney, but Darrell West of the Brookings Institution says the senator could help.

“Portman combines temperament and geography in a really appealing way” for Romney, says West, vice president and director of governance studies at Brookings. West believes Romney will ultimately choose Portman, but says comments by Romney’s wife, Ann, indicate it could be someone else as well. In an interview with CBS News in July, Mrs. Romney cited as one important factor the “same personality type” as her husband. Read more on Political Watch blog.

“I think that Pawlenty and Portman fit that bill, as does Rep. [Paul] Ryan from Wisconsin,” West says.

Rubio addresses the American Conservative Union's annual conference in February. Reuters

Marco Rubio: The 41-year-old, telegenic Cuban-American Rubio has several things going for him, but a number of factors work against him.

First, as a Floridian, he’s in a position to help Romney carry what is one of the most important swing states.

“Putting Rubio on the ticket might help there marginally,” says Abramowitz. “Even a very small bump [in Florida] could make the difference.” Florida, home to 29 electoral votes, is indeed close. Obama pulls in 47% of the vote to Romney’s 45.6%, according to the newest RealClearPolitics average of polls.

As a high-profile Hispanic lawmaker, Rubio could help Romney with voters in other Latino-heavy states, including New Mexico and Nevada.

Rubio would also offer an alternative on the sensitive issue of immigration: The Florida senator was readying his own version of the DREAM Act earlier this year before Obama in June announced that younger illegal immigrants brought to the U.S. as children and without criminal records wouldn’t be deported. Romney and Rubio criticized the move, saying it makes a long-term solution harder to achieve.

Then there is the question of readiness — what University of Virginia political scientists call the “Too fresh a fresh face?” issue and Goldstein characterizes as the “Is he ready for prime time?” question. Pawlenty and Portman have deep experience at high levels, but Rubio is in his first Senate term. Would that count against him as a potential successor to Romney? If Romney believes so, Rubio is probably out of the running.

Rubio is the only candidate that Romney has confirmed is being considered for the job of vice president.