It was just a matter of time. Charlie Crist is becoming a Democrat.

Crist — Florida's former Republican governor who relished the tough-on-crime nickname "Chain Gang Charlie" and used to describe himself variously as a "Ronald Reagan Republican" and a "Jeb Bush Republican" — on Friday evening signed papers changing his party from independent to Democrat.

He did so during a Christmas reception at the White House, where President Barack Obama greeted the news with a fist bump for the man who had a higher profile campaigning for Obama's re-election this year than any Florida Democrat.

The widely expected move positions Crist, 56, for another highly anticipated step: announcing his candidacy for governor, taking on Republican incumbent Gov. Rick Scott and an untold number of Democrats who would challenge him for the Democratic gubernatorial nomination.

"I've had friends for years tell me, 'You know Charlie, you're a Democrat and you don't know it,' " Crist, a career-long populist, recounted Friday night from Washington, D.C.

Crist has been registered with no party affiliation since the spring of 2010, when his Republican candidacy for U.S. Senate was fizzling against Republican upstart Marco Rubio. Since losing that race, he has been steadily inching toward the Democratic Party, first when his wife, Carole, switched her affiliation to Democrat and later when he threw himself into Obama's re-election campaign, earning a prominent speaking slot during the Democratic National Convention in Charlotte, N.C.

Critics from both parties sniff that Crist is merely looking for an avenue back into public office and is willing to throw out his principles to achieve the goal.

"Charlie Crist has the ability to meld into any character — from 'Chain Gang Charlie' to sympathetic 'Man of the People' — there is seemingly no role that he can't play," said one recent release from the Florida GOP, which has been blasting Crist regularly in anticipation of him running for governor as a Democrat.

Crist has been consistently opposed to taxes and gun control laws, but in many respects his record is appealing to Democratic activists and donors alike.

He has been a strong supporter of higher pay for teachers. He works for a leading trial lawyer. He was a leading advocate for civil rights as governor and attorney general. And though he describes himself as "pro-life," his voting record in the Legislature was mostly in favor of abortion rights. He has long been more of a populist than a pro-big business Republican.

"What changed is the leadership of the Republican Party," Crist said in a phone interview Friday night. "As I said at the convention, I didn't leave the Republican Party, it left me. Whether the issue was immigration, or education, or you name it — the environment. I feel at home now."

Charlie and Carole Crist had a tour of the White House on Thursday and ran into Valerie Jarrett, a senior adviser to the president. He mentioned that he was thinking of signing papers to become a Democrat the next night at a White House reception.

"She said, 'You're always welcome in my party. I think it's a great idea,' " recounted Crist.

The Obama administration has long had a strong relationship with Crist, who was one of the few high-profile Republicans to enthusiastically endorse the $700 billion stimulus package that he said helped keep Florida teachers and emergency workers employed and could have funded a high-speed rail initiative in Florida had Gov. Scott not rejected the money.

Crist is no shoo-in to win the Democratic gubernatorial nomination.

Other prospective candidates include former chief financial officer and 2010 gubernatorial nominee Alex Sink, Orlando Mayor Buddy Dyer, state Sen. Nan Rich of Weston and former Miami-Dade Commissioner Jimmy Morales.

"Crist was an absolute warrior for President Obama and as a result, many of the party activists I talk to are willing to welcome him with open arms, and there is no question he is an extremely viable candidate for governor, though I don't think that beating Alex Sink in a primary is any kind of sure thing," said Democratic strategist Steve Schale, who worked on Sink's campaign as well as both of Obama's Florida campaigns.

Sink, like Crist, has not committed to running for governor again, but in a Political Connections interview airing Sunday on Bay News 9 said Crist's potential candidacy would have little or no effect on her thinking.

"I don't go anywhere without strangers walking up to me on the street and asking me, and sometimes begging me and sometimes crying about their desire to see me run again," Sink said in the interview airing Sunday at 11 a.m. and 8 p.m. "If I run I'm in it to win, no matter who the other comers are."

Adam C. Smith can be reached at asmith@tampabay.com.