Artur Davis and Charlie Crist star in 'The Party-switchers Game'

Political conventions juggle two competing objectives: firing up the party faithful, and presenting an appealing face to undecided voters who are only beginning to tune in to the presidential race. This year's political conventions will both feature prime speaking roles for people who can help their parties achieve those dual goals: party switchers.

It’s OK, party switchers announce to the world, to change your mind and join the other team. For the home team, partisan defectors offer the fulfillment of a fantasy held by every die-hard politico since seventh-grade civics: convincing someone of another political persuasion to join your side.

For undecided voters, the switchers give a bipartisan sheen to a political party amid what is perhaps the most partisan affair of the year.

On Tuesday night, the GOP will feature former Democratic Rep. Artur Davis of Alabama. Mr. Davis will take the podium just a few speakers before the much-heralded address of Mitt Romney’s wife, Ann, and the keynote speaker, New Jersey Gov. Chris Christie.

For Republicans, having Mr. Davis at their dais in 2012 is particularly sweet: He seconded Barack Obama’s nomination at the Democratic National Convention in 2008.

Davis was a conservative Democrat during his tenure in the House, where he was among those who voted against Mr. Obama’s signature health-care reform law. However, he all but abandoned the party after he came up short in the Democratic gubernatorial primary in his home state.  

After Davis's speech, Republicans will lay on the “it’s OK to change your mind” theme with a testimonial video about three voters who supported Obama in 2008 but have shifted to Mitt Romney in 2012.

But Republicans aren't the only ones getting in on the switcheroo fun. Former Republican Gov. Charlie Crist of Florida endorsed Obama on Sunday and will speak at the Democratic National Convention next week.

And in the video tit for tat, the liberal "super PAC" Priorities USA released an advertisement featuring a Massachusetts small-business owner named Olive Chase, who supported Mr. Romney for governor but supports Obama for president.