As polls show the presidential race neck and neck in mid-July, the choice of a vice presidential running mate looms as crucial for likely Republican nominee Mitt Romney -- and possibly for President Obama as well.

The 2008 GOP nominee John McCain illustrated what not to do when he chose little-known and ill-prepared Alaska Governor Sarah Palin, and that clearly helped turn a close race into a decisive win for Barack Obama.

This time, Mr. Romney seeks to at least do no harm in his choice, but Republicans also hope he selects someone who can carry a key state or help with a specific voting group -- such as women, African Americans or Hispanics.

Or he could choose a non-offensive, if unexciting VP nominee, such as former Minnesota Governor Tim Pawlenty or Ohio Senator Rob Portman.

Those with obvious appeal, like blunt-spoken New Jersey Governor Chris Christie, or former Secretary of State Condoleezza Rice, a woman and African American, or youthful Florida Senator Marco Rubio, who could help the GOP win that state and Hispanic votes, also carry baggage that might hurt as much as help Mr. Romney.

Ms. Rice is pro-choice, which would set off explosions among social conservatives; Governor Christie is sometimes too volatile and might upstage Mr. Romney, and Mr. Rubio is Cuban, which may or may not help the ticket with other Hispanics. The senator also is relatively new on the national scene and has made some comments that might not play well in other sections of the country.

The president, however, seemingly would see only an upside if he were to choose his former nomination rival, Secretary of State Hillary Clinton instead of current VP Joe Biden.

That estimate could change, but at the moment it seems highly likely that the former first lady and New York senator, whose popularity is highest among Democrats, would help with women and with working class voters in states like Wisconsin, Michigan, Pennsylvania, Ohio, and Virginia -- all considered battleground states in the election.

Switching isn't something Mr. Obama likely would do, as he seems to prefer trusting in his own instincts and abilities and might be -- like most presidents -- averse to sharing the spotlight. But consider that Franklin Roosevelt, who won four presidential elections, changed vice presidents like shoes -- three times. And consider that the difference between winning a second term and losing one is, in historical terms, the equivalent of the Grand Canyon.

Just ask Jimmy Carter or George H.W. Bush.