Former Florida Republican governor Charlie Crist , who endorsed President Obama in a Tampa Bay Times op-ed on Sunday, will speak at the Democratic National Convention next week, ABC News has learned.

A Democratic official, speaking on condition of anonymity, said Crist's appearance in Charlotte, N.C., would be a testament to a spirit of openness and bipartisanship at the convention.

"The Democratic convention will be about bringing people together to continue the progress we've made in rebuilding our economy from the middle out, not the top down," the official said.

"Gov. Crist can personally speak to this, and contrast the president's vision with Mitt Romney's, which caters to the most extreme elements of the Republican Party and undermines the middle class."

Republicans criticized Crist as a turncoat and said his endorsement would not hold much influence with Sunshine State voters.

"Gov. Crist's endorsement adds one more vote in Florida and that's it," said Virginia Gov. Bob McDonnell, chair of the Republican Platform Committee, on ABC's "This Week."

Florida GOP chairman Lenny Curry said, "Crist has demonstrated, yet again, that his political ambition will always come first."

In 2010, Crist lost the Florida Republican primary for U.S. Senate, switching affiliations to run as an independent in the general election. He lost that race to the young, Tea Party-backed state rep Marco Rubio.

"[A]n 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," Crist wrote in the Times, explaining his support for Obama.