Chip Litherland for The New York Times

TAMPA, Fla. — Charlie Crist, the former Republican governor of Florida who ran for the Senate as an independent, will speak at the Democratic National Convention next week, taking yet another step away from his erstwhile party.

Ben LaBolt, an Obama campaign spokesman, confirmed Mr. Crist’s role just a day after he endorsed the president for re-election. The chairman of the Florida Republican Party, Lenny Curry, derided that endorsement as Mr. Crist’s effort to “shed his skin for a political comeback.”

Mr. Crist, who hails from nearby St. Petersburg, was once seen as a rising Republican star whose endorsement was sought by several of the party’s presidential candidates in 2008. But he fell out of favor in 2009 when he embraced President Obama’s economic stimulus efforts, and he ran for the Senate as an independent in 2010 after it appeared he would lose the Republican primary to Marco Rubio, the Tea Party-backed candidate. Mr. Rubio, now the junior senator from Florida, is scheduled to introduce Mitt Romney at the Republican National Convention here on Thursday.

Convention speakers from the opposite party are common trophies, and have been sought more aggressively in recent years as the parties try to highlight their broad appeal.

Mr. Crist was viewed as a contender to be Senator John McCain’s running mate in 2008, but he is still not the most high-profile poaching in modern convention history. That title belongs to Senator Joseph I. Lieberman, who ran as Al Gore’s running mate in 2000. After losing the Democratic primary in Connecticut in 2006, Mr. Lieberman then succeeded where Mr. Crist failed by winning his Senate race as an independent.

Mr. Lieberman’s further split with Democrats over foreign policy led him to address the Republican convention in 2008.