Florida Sen. Mel Martinez said Friday he is resigning to return to Florida and his family.

The first-term U.S. senator made a formal announcement at a 3 p.m.news conference in Orlando, saying he promised he wouldn't "simply warm a seat."

He told his staffers at a meeting Friday morning that will not return to Congress after the August recess. 

"My priorities have always been my faith, my family and my country and at this stage in my life, and after nearly 12 years of public service in Florida and Washington, it's time I return to Florida and my family," Martinez said in a letter to friends and supporters, obtained by FOX News.

"So today I'm announcing my decision to step down from public office, effective on a successor taking office to fill out the remainder of my term."

Martinez, 62, the only Hispanic Republican in the Senate, announced last year that he wouldn't seek re-election in 2010.

The news that Martinez would not serve out his term drew immediate criticism from Democrats.

"Republicans seem to have a problem fulfilling their oaths of office," Democratic Senatorial Campaign Committee spokesman Eric Schultz said in a written statement. "This is the largest number of retirements for one party in the Senate in the past 80 years."

"The country faces enormous challenges right now, and voters are watching who is up to the task," he said.

Rumors had been swirling for months that Martinez would resign early. Several sources told FOX News that the senator wants to return to Florida to help resolve some family issues.

His resignation will allow Gov. Charlie Crist, who is running for the seat, to appoint a replacement. Sources told FOX News that Crist, a Republican, will appoint a placeholder and let the primary unfold as usual. Crist is not likely to appoint himself because it could alienate voters.

It is unclear whether Martinez would endorse a primary candidate.

With Democrats holding a filibuster-proof 60-seat majority in the Senate, every seat is critical for Republicans. The Florida race has captured national attention because it is one of several contests in which the GOP must find strong candidates to run.

Martinez announced in December that he would not seek a second term next year, citing a desire to spend more time with his family.

He denied that he faced difficult re-election prospects in a state that President Obama won in 2008. But he has struggled to boost public support because of his close ties to President George W. Bush and his efforts to push an immigration bill that was unpopular with many Republicans.

Martinez was elected in 2004 after serving as the U.S. secretary of housing and urban development during the Bush administration. He served as general chairman of the Republican National Committee for 10 months before resigning in October 2007.

FOX News' Major Garrett and Trish Turner and The Associated Press contributed to this report.