Tampa may have escaped the worst of tropical storm Isaac, but Republicans arrived for their convention to find another kind of tempest brewing over the direction of their party.

Florida's former Republican governor, Charlie Crist, stole almost as much of the convention's thunder as Isaac by formally endorsing President Barack Obama over GOP nominee Mitt Romney on Sunday. He lashed out at the GOP for moving so far to the right that moderates like him have been left with no other choice but to quit the party.

"An element of their party has pitched so far to the extreme right on issues important to women, immigrants, seniors and students that they've proven incapable of governing for the people," Mr. Crist wrote in a Sunday op-ed in the Tampa Bay Times. "The truth is that the party has failed to demonstrate the kind of leadership or seriousness voters deserve."

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Mr. Crist's critique comes as Republicans prepare to formally adopt a platform that, more than any other in recent history, bears the imprint of social conservatives and Tea Partiers. For the first time, the GOP program promises to subject the Federal Reserve to a full congressional audit, a move critics say would threaten its independence.

Mr. Crist is among a chorus of disaffected Republican leaders who have either left the GOP since the rise of the Tea Party movement or distanced themselves from the party.

Former senator Bob Dole, the 1996 GOP presidential nominee, said in a weekend interview that he worried the party was not doing enough to appeal to "mainstream" voters and attract support among Hispanics and young people.

"We have got to be open," he told The Daily Telegraph. "We cannot be a single-issue party or single-philosophy party…There's a big split in our party. There's this undercurrent of rigid conservatism where you don't dare not toe the line."

Indiana Senator Richard Lugar, who lost his state's May GOP primary to a Tea Party candidate, also took issue on Monday with the party's unwillingness to compromise.

"Independent voters who decide many elections are looking for competence, seriousness of purpose and trustworthiness. They are looking for leaders who will not be dissuaded from compromises that might be necessary to govern effectively," Mr. Lugar wrote in an op-ed published in USA Today.

The comments of Mr. Dole and Mr. Lugar echo those of Mr. Crist's successor as Florida governor, Jeb Bush, who criticized the harsh Republican rhetoric on illegal immigration. Most Hispanic voters, who form the fastest growing block of the electorate, have a relative, friend or neighbour who is in the country illegally.

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"You can't ask people to join your cause and then send a signal that you're really not wanted. It just doesn't work," Mr. Bush said on Meet the Press on Sunday.

Unlike Mr. Crist, Mr. Bush is in for the long haul. Most observers expect him to seek the GOP nomination in 2016 if Mr. Romney loses in November.

In Mr. Crist's case, however, it is as much a question of whether he quit the GOP or was shown the door. Trailing Marco Rubio in the 2010 GOP Senate primary, he opted to run as an independent candidate. Despite winning the support of many moderate Republicans and centrist Democrats, he still lost the November election to Mr. Rubio.

Even so, Mr. Crist's endorsement may give Mr. Obama a boost in a critical swing state. The Real Clear Politics average of presidential polls in Florida shows Mr. Obama and Mr. Romney in a dead tie, each with the support of 46.3 per cent of voters.

"For Charlie Crist to pull this Obama stunt while Florida faces a hurricane only proves Charlie Crist cares about only one thing: Charlie Crist," retorted Lenny Curry, the GOP state chairman, said in a Monday interview with the Tampa Bay Times.

While other top Republicans accused him of abandoning his principles, Mr. Crist insisted to the contrary: "I haven't changed. The Republican Party has changed."