Geoff Duncan was the Republican lieutenant governor of Georgia when President Trump lost the state in 2020 and tried to subvert the outcome. On Tuesday, Mr. Duncan announced that he was running for governor as a Democrat — the culmination of a political evolution that began in the tumultuous months after the election five years ago.

Mr. Duncan could prove to be a wild card in an election next year that will be a crucial test for Georgia’s relatively new status as a swing state. Gov. Brian Kemp, a popular Republican, is prevented by term limits from seeking re-election.

In a video announcing his run, Mr. Duncan boasted of being a target of Mr. Trump’s ire, highlighting a social media post in which the president called him a “total loser.”

“This attack from Donald Trump is a badge of honor for me,” he said in the video. “It means I’m doing something right.”

In the state’s Democratic primary, Mr. Duncan will face Keisha Lance Bottoms, the former Atlanta mayor who announced her candidacy in May, and Jason Esteves, who until recently was a state senator from Atlanta. Mr. Esteves resigned from the Senate last week to focus on the governor’s race.

On the Republican side, Burt Jones, Mr. Duncan’s successor as lieutenant governor, and Chris Carr, the state attorney general, are the two leading candidates.