The media is rife with speculation about Marco Rubio, Paul Ryan, Chris Christie, Susana Martinez, Bob McDonnell or Rob Portman as possible running mates for Mitt Romney. But Romney's choice is likely to evolve, in ways unforeseeable today, over the next four months.

In weighing the reliability of analyses that tell you Romney is most comfortable with Ryan, the Wisconsin congressman and budget policy wonk, or that Rubio, the young Cuban-American freshman senator from Florida, is the linchpin to the Latino vote, consider these examples from recent elections:

In April 2000, the leading Democratic contenders were supposed to be Sens. John Kerry of Massachusetts or John Edwards of North Carolina. The nominee, Vice President Al Gore, did pick a Democratic senator: Joe Lieberman of Connecticut.

There was a longer list of Republican contenders that year, though Pennsylvania's popular governor, Tom Ridge, shot to the top after former congressman and defense secretary Dick Cheney was tapped to head a search committee. Right before the summer convention, George W. Bush instead selected Cheney.

Four years ago, Sen. Joe Biden of Delaware was considered one of the leading contenders. But on the Republican side, John McCain couldn't have picked Sarah Palin out of a lineup in April 2008. He barely knew who she was when he selected her four months later.

Then there's the supposed electoral weight some candidates bring: Rubio in Florida or Portman in Ohio. Yet over the past 40 years and 10 presidential elections, no running mate has made the difference in carrying a state. (Vice President Walter Mondale, running with President Jimmy Carter in 1980, is a debatable exception.)

Devotees of the Electoral College love to point to Lyndon Johnson of Texas winning the presidency for John F. Kennedy in 1960. Johnson almost surely carried his home state for Kennedy, but JFK would have won in any case.

And there have been highly praised vice presidential choices who couldn't make a difference even in their home states: Texas Democrat Lloyd Bentsen in 1988 and New York Republican Jack Kemp in 1996. So much for that theory.

That isn't to say that Romney's selection is unimportant. It will help shape what the campaign hopes is a reset -- or a shaking of the Etch A Sketch -- as the nominee faces a different electorate. It can send a message.

Both Bush, with Cheney, and Obama, by picking Biden, reassured voters about their relative inexperience. Bill Clinton and Gore symbolized a new generation ready to take charge after the fall of communism. Ronald Reagan made a bow to the center by selecting George H.W. Bush.

The chief consideration, people who've been through the process agree, is to do no harm. Running mates can help marginally; they can hurt substantially. That's why the Romney team needs to ask hard questions of the more appealing choices.

Rubio's telegenic youth and his ethnicity provide an attractive balance to Romney's awkward, corporate persona. Yet the 40-year-old Florida lawmaker is inexperienced, hasn't impressed Washington heavyweights with his substance or readiness to be president, and still faces some controversies in his home state.

Ryan, 42, is the poster child for the conservative economic establishment, a policy expert and heir-apparent to the late Jack Kemp. He's also never run outside his small congressional district and has never shown any of Kemp's passion for equal opportunities and civil rights. Ryan's fiscal plan could be politically perilous: He won't say how he would pay for his $4.6 trillion tax cuts, which principally go to the wealthy.

The economic-conservative wing has a big bullhorn in the party. That might deter Romney from considering Mike Huckabee, the ex-Arkansas governor and 2008 presidential aspirant with an economic-populist streak that appeals to evangelicals.

Other candidates who might be acceptable to evangelicals and economic conservatives could complicate Romney's problems with women voters; such is the case with former Pennsylvania Sen. Rick Santorum, who has said he'd like contraception to be outlawed, or Virginia Gov. McDonnell, who signed legislation requiring women to undergo an ultrasound before an abortion.

Romney could turn to women such as Govs. Martinez of New Mexico or Nikki Haley of South Carolina. Both have the same experience -- or lack thereof -- as then-Alaska Gov. Palin did four years ago; that memory is too fresh for most Republicans.

Governors with a little more seasoning, such as Louisiana's Bobby Jindal or Brian Sandoval of Nevada, have other problems. Sandoval favors abortion rights, a disqualifier for many Republicans; Jindal signed a measure encouraging the teaching of creationism in the Louisiana schools, which wouldn't play well with independent suburbanites.

Most important will be the conditions of the race. This summer, will Romney be 10 points behind or a few points ahead? Will economic recovery be stalled or taking root? Will the GOP conservative base's hatred of Obama overcome weak enthusiasm for a Mormon nominee who is a suspected moderate?

The most often-cited vice presidential candidates are above. The odds are 4 to 1 that on Wednesday, Aug. 29, a person who isn't on this list will be anointed at the Republican convention in Tampa.