WASHINGTON — This is Artur Davis' job now, the work he hopes will resurrect his political career. Wear a suit. Speak to strangers. Explain that what had been some of the most important causes of his life — a political party and a president — turned out to be mistakes.

"How many of us believed, four years ago, that Barack Obama was not just a politician?" Davis, a former four-term congressman, asked Mitt Romney supporters in Arlington, Va., on Wednesday.

"We may not have the power to stop it," Davis said of President Obama's campaign. "But the American people have the power to punish it."

Four years ago, Davis was onstage at the Democratic convention: a fast-rising Democratic congressman from Alabama, so close to Obama that he provided the official "second" for Obama's nomination.

On Thursday, the Republican Party said he would be a "headliner" at its convention in Tampa, where he will be one of Obama's most prominent African-American critics.

In between those two big convention moments, Davis was bounced out of politics after a disastrous run for governor of Alabama. After the loss, he abandoned the Democratic Party, saying it had drifted too far left.

Even before Thursday's news, Davis, 44, had already made a remarkable leap.

He has been out of Congress since early 2011. He left Alabama and moved with his wife into a new high-rise in Pentagon City, in Arlington.

Davis, a graduate of Harvard and Harvard Law, said he moved to Virginia to join a Washington, D.C., law firm, not to run for office. But last year, he called Democratic consultant Mo Elleithee.

"I remember him specifically saying, 'I still have the political bug. And I'm thinking about running in Virginia,' " Elleithee recalled Thursday.

Elleithee gave him bad news: running for Congress in Northern Virginia would mean taking on entrenched Democrats such as James Moran and Gerald Connolly. Davis, as a newcomer, would stand little chance.

Then on May 29 this year, Davis typed words that would change his life. "If I were to leave the sidelines, it would be as a member of the Republican Party," Davis wrote on his blog. "Wearing a Democratic label no longer matches what I know about my country and its possibilities."

By May 30, Davis was on the Fox News Channel. "He was once called 'the Obama of Alabama,' " host Neil Cavuto told viewers.

When Obama ran for president, Davis was one of the first outside Illinois to endorse him. At the convention in Denver, Davis' seconding speech celebrated Obama's progress — and his own. He described watching the 1988 Democratic convention on a motel-room TV, after his family had been evicted from home. Now he was on the televisions and on the stage.

In 2010, Davis ran for Alabama governor. But he also voted against the president's health-care overhaul in Congress, saying the law was too expensive and brought too much government interference.

He lost to a more liberal, white candidate by 24 points.

Now, a few weeks after declaring himself a Republican, Davis has become an energetic and versatile spokesman for his new party.