WASHINGTON - Senator Mel Martinez, a native Cuban and a former chairman of the Republican Party, stunned supporters on Friday by informing them that he plans to resign from Congress, party aides said.

Martinez, 62, who earlier said he would not run for a second Senate term next year, scheduled a news conference in Orlando, Florida, at 3 p.m. EDT to make an announcement.

In a letter to friends, a copy of which was obtained by Reuters from a supporter of the senator, Martinez said he planned to step down after a replacement is named.

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 wrote.

Before being elected to the Senate in 2004, Martinez headed the U.S. Department of Housing and Urban Development in the George W. Bush administration and served as a Florida mayor.

He stepped down in 2008 as chairman of the Republican Party after holding the post for about 10 months.

Martinez's resignation likely would not change the balance of power in the Democratic-led Senate since Florida Governor Charlie Crist is certain to name fellow Republican to replace him.

Crist earlier announced that he plans to run next year for the seat.

Martinez came to the United States from Communist Cuba as a boy and used his post as a U.S. senator to speak out against the oppression in his homeland.

On Thursday, Martinez broke ranks with most fellow Senate Republicans when the Democratic-led Senate confirmed Sonia Sotomayor as the first Hispanic on the U.S. Supreme Court. Martinez is the only Hispanic Senate Republican.

The Florida Democratic Party was quick to react to news of Martinez's plans to resign.

Chair Karen Thurman called on Crist to appoint an independent caretaker before an election can be held next year to fill Martinez's seat.

U.S. Senate seats should not be political footballs, Thurman said in a statement.

In 2010, every statewide office in Florida will be wide open with no incumbents running for re-election, Thurman said. Democrats are working to seize our historic opportunity.

(Additional reporting by Jane Sutton in Florida; editing by Bill Trott)