This is Artur Davis’s job now, the work that 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 County’s Ballston neighborhood on Wednesday. The Romney people said nothing, but Davis kept on: This was his story, not theirs.

“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 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 attempt to mimic the Obama playbook in a run to become governor of Alabama. After the loss, he abandoned the Democratic Party, saying it had drifted too far left.

Davis’s new life reveals the benefits and burdens that come with being a celebrity turncoat.

“You have a converted sinner who’s standing in front of you right now, and I thank you for letting me stand here,” Davis told a tea party audience in Falls Church in July. At the front of that room, Davis seemed to marvel at his rapid success as an ex-Democrat.

“I used to go to the Baptist church in Birmingham,” he said. “And Baptists are good folks, but they won’t let nobody preach on week one, or month one, like y’all will.”

Even before Thursday’s news, the 44-year-old Davis (whose first name is pronounced “Ar-TOOR”) had already made a remarkable leap out of his political grave.

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.

Davis said he moved to Virginia to join a D.C. law firm, not to run for office. But last year, he called Democratic campaign 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 P. Moran Jr. and Gerald E. Connolly. Davis, as a newcomer, would stand little chance.

Davis says he was just surveying the landscape in his new state: “I’m still not in a decision-making mode when it comes to running for office.”

A few months later, on May 29, 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.”

Davis says that, as a center-right politician, he has felt increasingly at odds with the Democratic support for higher taxes and affirmative action.

“This is a two-team sport,” he said. “You do have to choose sides.”

Amardo Wesley Pitters, one of two best men at Davis’s wedding in 2008, said Davis did not intend to let the 2010 defeat end his career.

“ ‘It’s not over. It’s not over until I say it’s over,’ ” Davis told him, Pitters recalled. Davis says the conversation never happened: “It’s unfortunate to see people who were once good friends literally invent conversations.”

After Davis announced his party switch, he said, nothing happened for a full day. No one noticed. But Artur Davis the ex-Democrat was something else entirely. By May 30, Davis was on the Fox News Channel. “He was once called ‘the Obama of Alabama,’ ” host Neil Cavuto told viewers.

Davis’s life story is a kind of Deep South parallel to Obama’s. Davis also lived most of his childhood without his father, who left when Davis was 2. He went from Montgomery’s Jefferson Davis High School to Harvard, where he got both a college and a law degree, graduating from law school two years after Obama.

After law school, Davis worked as a prosecutor, then challenged a veteran Democrat for the congressional seat representing Alabama’s Black Belt. In 2000, he lost. But in 2002, he won.

When Obama ran for president, Davis was one of the first outside Illinois to endorse him. At the convention in Denver, Davis’s 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 their home. Now he was on the televisions and on the stage.

“As our next president has said, ‘From the places in America where people hurt, to the places where people dream, our time is now,’ ” Davis said, quoting Obama. “Our time is now.”

But if Davis was right about Obama’s political moment, he was wrong about his own.

In 2010 he ran for Alabama governor, telling journalist Gwen Ifill that Obama was his model: “Ultimately, I think we can run a campaign that just doesn’t get too tied down to the anchor of race.” But Davis also had voted against the president’s health-care overhaul in Congress, saying that the law was too expensive and brought too much government interference.

Politically, that was a disaster — a rejection of the first black president’s signature law in a state where black voters can swing the Democratic primary. Davis lost to a more liberal, white candidate by 24 points.

“It is impossible to be a credible black politician, [a] congressman, and lose a Democratic primary for governor,” said John Anzalone, a longtime Alabama pollster. “And he somehow found a way to do it.”

Now, just a few weeks after declaring himself a Republican, Davis has become an energetic and versatile spokesman for his new party. On Wednesday, speaking on Romney’s behalf in Ballston, he attacked Vice President Biden for his comment, to a largely black audience, that Republicans wanted to “put y’all back in chains.”

“Every African American in that audience knew who the ‘y’all’ was, and every African American in that audience knew what buttons he was trying to push by talking about ‘chains,’ ” Davis said, to a largely white audience.

Davis says he is just one of millions of disaffected centrists who voted for Obama only to be left disillusioned by his failures as president.

But in front of some audiences, Davis sounds like something else: a strident conservative, frightened of Obama's plans for America.

“When the story of how freedom was won in 2012 is told — when the story of how we reclaimed our faith and our traditions is told — let them say that the truth-tellers, the people who struck the final blow, were people in the great state of VIRGINIA!” Davis told a GOP group in Burke earlier this year. “Let’s be the ones to put Mitt Romney over the top, to save this country.”

In Alabama, some don’t understand the turn Davis has taken. Byron Perkins, a lawyer who was Davis’s other best man, said he had called his friend for an explanation.

“Quite frankly, I can’t accept it, until he sits down and explains to me why he’s doing it,” Perkins said.

But Davis said he’s happy with his decision to switch, despite vitriol from people who knew him before.

“In Byron’s worldview, an African American becoming a Republican is a sign of deep trouble,” Davis said. “I don’t spend a lot of time trying to persuade people of political choices that I know they don’t share.”