Miami - Republican Senator Marco Rubio of Florida announced on Wednesday he will run for re-election, a turn-about from his pledge to either win the US presidency this year or return to private life.
The move would make the high-profile Rubio the immediate frontrunner in the race and provides a boost to Republican Party efforts to block Democrats from snatching the seat in a key battleground state.
"Control of the Senate may very well come down to the race in Florida," he said in a lengthy statement explaining his decision.
Rubio noted that Supreme Court nominations, the future of the "disastrous" Iran nuclear deal, and economic and foreign policies could be determined by the outcome of the Florida race.
"The stakes for our nation could not be higher," he added.
Rubio, 45, said he was eager to "act as a check and balance on the excesses of a president", adding he would eagerly fulfil that congressional role regardless of whether fellow Republican Donald Trump or Democrat Hillary Clinton is elected in November.
Rubio has significant policy differences with Trump and he said the prospect of a Trump presidency remained "worrisome".
"If he is elected, we will need senators willing to encourage him in the right direction, and if necessary, stand up to him. I've proven a willingness to do both," Rubio said.
He immediately earned an endorsement from Ted Cruz, another US senator born to Cuban immigrants and who ran unsuccessfully for the presidency this year.
"Marco is a tremendous communicator and a powerful voice for the American Dream," Cruz said in a statement.