Florida Republican Marco Rubio, a former state House Speaker, announced Tuesday morning that he is a candidate in the 2010 race for the U.S. Senate seat left open by retiring one-term Republican Mel Martinez.

Rubio, with his candidacy announcement, defied the conventional wisdom that the field for the Senate contest was frozen pending Republican Gov. Charlie Crist‘s decision on whether he will enter the race. Crist, who has had strong approval ratings since his election in 2006, has been mulling whether to run for the Senate or seek a second term as governor, and has said that he is waiting until the state legislature adjourns its annual session this month before going public with his choice.

Rubio — who, like Martinez, is a prominent member of Florida’s politically potent Cuban-American constituency — is seen by a number of Republicans as a party rising star. He received mention as a possible Senate candidate following Martinez’ retirement announcement last December, and emerged in the top tier of prospective contenders in January after former Gov. Jeb Bush stated he would not enter the Senate race.