U.S. Sen. Connie Mack's decision not to seek re-election in 2000 gives Democrats a shot at a seat that was regarded as a lock for the two-term Republican.
No reason was given for the decision, but an adviser to Mack said he is resigning because he believes it's time to move on to a new challenge.
Mack, 58, is expected to make a formal announcement in Washington on Monday about his future, deputy press secretary Bethany Rogers said Friday.
Rumors had swirled in recent weeks that Mack might leave the Senate. He was first elected in 1988 and easily defeated Hugh Rodham, Hillary Rodham Clinton's brother, in 1994 with 71 percent of the vote.
Gov. Jeb Bush tried last week to persuade Mack to remain in the Senate. The governor is "surprised and disappointed," Bush spokesman Cory Tilley said. "But he understands Sen. Mack's reasons."
Republicans now hold a 55-45 edge in the Senate, but must defend 19 seats next year, compared with 14 for the Democrats. The 19 GOP seats include many held by first-termers who were pulled into office on the strength of the GOP landslide in 1994.
Potential candidates for Mack's seat include U.S. Reps. Mark Foley, a Republican, and Peter Deutsch, a Democrat. Others include state Insurance Commissioner Bill Nelson, a former Democratic congressman, and Republican U.S. Rep. Bill McCollum, whose profile was raised as a prosecutor during President Clinton's impeachment trial.
Another politician who might run is former Republican state Sen. Charlie Crist, who lost to U.S. Sen. Bob Graham last year.
Mack's razor-thin victory over Democrat Buddy MacKay in 1988 began a series of election victories that catapulted Republicans into control of Florida politics.
Mack won re-election by a resounding margin four years ago when the GOP gained control of the Florida Senate. The Republicans gained control of the House two years later and crowned off the sweep of state politics when Bush was elected governor last November.