Florida Republican Gov. Charlie Crist announced Tuesday that he will run in 2010 for the state’s open Senate seat and forgo a bid for re-election as the state’s chief executive — a move that has prompted CQ Politics to change its ratings on both the governor’s and Senate races.

The race for the Senate seat, rated Tossup before this morning’s announcement, is now rated Leans Republican. Crist has enjoyed strong approval ratings in Florida polls since his election as governor in 2006 — a leading factor in the effort by top Republican officials to persuade him to run for the seat left open by first-term Republican Sen. Mel Martinez , who announced Dec. 2 that he would not seek re-election.

The governor’s race, assumed to be Crist’s for the asking had he decided to seek a second term, is now up for grabs. CQ Politics, which had rated the race Republican Favored, has changed the rating to Tossup.

Texas Sen. John Cornyn , who orchestrates the GOP’s national Senate campaign for 2010 as chairman of the National Republican Senatorial Committee (NRSC), issued an endorsement immediately after Crist released a written statement confirming his candidacy.

Also, Crist’s name is much more recognized statewide than any of the already announced or prospective Senate candidate — including Rep. Kendrick B. Meek , widely regarded as the early front-runner for the Democratic Senate nomination, and former state House Speaker Marco Rubio, who announced his candidacy last week and now faces a choice of challenging Crist for the nomination or switching to another race.

Rubio spoke with Cornyn by phone Monday night and came to Washington, D.C., Tuesday to meet with him. “Maybe there’s an alternative that would allow him to continue his desire to serve the people of Florida,” said Cornyn, who described his job, in part, as “trying to defuse potentially nasty primaries.”

But Rubio’s initial reaction showed he is not ready to back down. “Let the debate begin,” he proclaimed in response to Crist’s entry.

Crist’s announcement ends an extended wait for him to decide which of the two key statewide races he would enter. Many Republicans viewed Crist as their strongest Senate prospect after his predecessor as governor, Republican Jeb Bush, stated in January that he would not run. But Crist stuck to his determination to withhold any announcement until the Florida legislature adjourned for the year, which it did last Friday.

Crist, in his statement Tuesday morning, said, “As governor, each day I will continue to focus on fighting for all Floridians. And if I am given the honor of serving as their senator, I will take that fight for the people of this state I love so dearly to the halls of Congress.”

It remains to be seen whether Crist will have a clear field for the August 2010 Republican primary. Rubio, who is seen by supporters as one of the party’s rising stars, moved to pre-empt Crist’s expected announcement by declaring last week his own intention to seek the Senate GOP nomination.

Crist does have critics on the Republican right, because he endorsed President Obama’s economic stimulus bill, which drew almost unanimous Republican opposition in Congress, and because he is viewed as relatively moderate on social issues. Some conservatives might be angered if they view the party brass as trying to muscle Rubio out of the Senate race or otherwise stifle any attempt to generate a conservative challenge to him in the primary.

But it appears that the state and Washington establishments will rally around Crist and encourage Rubio to jump into race for the governor — in which he had previously indicated an interest, and for which the GOP is now in search of a top-tier candidate to replace Crist — or another statewide office, such as Attorney General.

NRSC head Cornyn wasted no time in establishing the national party establishment’s preference for Crist in the Senate race.