TALLAHASSEE, Fla. -- Former Florida Gov. Charlie Crist, who was elected the state's chief executive as a Republican and then ran unsuccessfully for the U.S. Senate as an independent, announced on Twitter that he's switching to the Democratic Party.

Crist's party switch reflects the growing concern among some moderate Republicans that their party has left them behind. In an interview with the Tampa Bay Times , Crist cited the Republican Party's shift to the right in recent years on a range of issues, including immigration, education and the environment.

The announcement Friday night fanned speculation that Crist would seek to regain his old job from Republican Gov. Rick Scott in 2014.

Crist sent out a tweet that said, "Proud and honoured to join the Democratic Party in the home of President @Barack Obama!"

The tweet included a photo of a smiling Crist and his wife Carole as he held up a Florida voter registration application. The Tampa Bay Times reports that Crist signed the papers changing his affiliation from independent to Democrat at a Christmas reception at the White House. President Barack Obama greeted the news with a fist bump.

"I've had friends for years tell me, 'You know Charlie, you're a Democrat and you don't know it,"' Crist told the newspaper Friday night.

Messages left for Crist by The Associated Press weren't immediately returned Friday night.

Crist was elected Florida governor in 2006 as a Republican. As he moved to run for the U.S. Senate in 2010, he faced a tough primary challenge from the right and bolted the Republican Party to run as an independent. He lost a three-way Senate contest in 2010 to Republican Marco Rubio.

Crist, 56, spoke at the Democratic National Convention in Charlotte, North Carolina, that nominated Obama for a second term and campaigned for his re-election. Obama ended up narrowly winning Florida.

Crist's decision to become a Democrat will increase speculation that he intends to challenge Scott, a former hospital chain CEO who has struggled with low favourability ratings since taking office. Crist has already criticized Scott for refusing to extend early voting despite pleas from U.S. Sen. Bill Nelson and other Democrats that it would result in unusually long lines.

But it is unlikely that Crist would get a clear path to the Democratic nomination. Former State Sen. Nan Rich has already jumped into the race and former Chief Financial Officer Alex Sink could run. Sink barely lost the 2010 governor's race to Scott. Some Democrats remain wary of Crist and even outgoing Florida Democratic Party chairman Rod Smith has joked that just because someone joins the congregation, "you don't make them the preacher."

Steve Schale, a Democratic political consultant who worked on Obama's Florida campaign, called a Crist a "viable Democrat" who "earned his stripes when he supported the president.

Republicans in recent weeks have already ramped up their criticism of Crist and have pointed out that in the past he was critical of Obama and once described himself as a Republican in the mould of President Ronald Reagan and Crist's predecessor as governor, Jeb Bush.

"Charlie Crist's first official act as a Democrat was to tell a lie about why he is now pretending to be one," the Florida Republican Party said in a statement early Saturday. "The truth is that this self-professed, Ronald-Reagan Republican only abandoned his pro-life, pro-gun, conservative principles in 2010 after he realized that Republicans didn't want to send him to Washington D.C. as a senator, especially after he proved he couldn't do the job as governor."