WASHINGTON -- Former Republican presidential candidate Marco Rubio announced Wednesday he will run for re-election to the Senate from Florida.

He reversed his retirement plans under pressure from GOP leaders determined to hang onto his seat and Senate control.

"In politics, admitting you've changed your mind is not something most people like to do. But here it goes," Rubio said. "I have decided to seek re-election to the United States Senate."

The 45-year-old first-term senator repeatedly had described his frustration with the slow-moving Senate and was expected to enter the private sector and prepare for another presidential run in 2020.

But he had been rethinking his plans, particularly after the massacre in Orlando and a pressure campaign led by Senate Majority Leader Mitch McConnell.

GOP leaders were concerned the Republican candidates who'd emerged for Rubio's seat were not up to the task of winning Florida amid the turmoil created by Donald Trump's candidacy.

Trump himself had gotten into the act, tweeting his encouragement to Rubio, whom he dubbed "Little Marco" during the presidential race, to run for re-election.

"Control of the Senate may very well come down to the race in Florida," Rubio said Wednesday, adding that the outcome also could determine the makeup of the Supreme Court and critical fiscal and economic policies. "The stakes for our nation could not be higher."

Rubio also said that "No matter who is elected president, there is reason for worry." After criticizing Democrat Hillary Clinton, Rubio went on to describe his disagreements with Trump, saying that "some of his statements, especially about women and minorities, I find not just offensive but unacceptable."

"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."

The announcement comes just days ahead of Friday's filing deadline for Rubio's seat.

Republican leaders welcomed the news. "While Marco is already in a strong position to win, Democrats are currently locked in a bruising primary that will produce a weak nominee and cost millions of dollars," said Sen. Roger Wicker of Mississippi, chairman of the National Republican Senatorial Committee.

Rubio's embrace by GOP leaders is a marked change from when he first ran for Senate six years ago. At that time, he was a tea party-backed upstart successfully challenging the establishment choice, then-GOP Gov. Charlie Crist. Now Rubio is the establishment candidate, leaving it to other Republicans to try to claim the insurgent mantle.

Two Republicans planning to run in the Aug. 30 primary -- Lt. Gov. Carlos Lopez-Cantera and Rep. Ron DeSantis -- bowed out Wednesday after hearing Rubio's news. A third, Rep. David Jolly, had already done so. But two wealthy first-time candidates, businessman Todd Wilcox and developer Carlos Beruff, remained in the race. Beruff lashed out at Rubio in a statement saying: "This isn't Marco Rubio's seat; this is Florida's seat. The power brokers in Washington think they can control this race. They think they can tell the voters of Florida who their candidates are. But the voters of Florida will not obey them."

Democratic leaders' preferred candidate is Rep. Patrick Murphy, but Murphy must first get through his own primary against controversy-stoking liberal firebrand Rep. Alan Grayson.

Murphy issued a statement attacking Rubio for running for re-election after saying he wouldn't, and missing Senate votes to run for president. "Marco Rubio abandoned his constituents, and now he's treating them like a consolation prize," Murphy said.

Some leading Republican donors and outside groups had been reluctant to invest in the race unless Rubio ran, and quickly announced their intent to get behind him and seek to turn Democratic divisions to their advantage.

"We are 100% IN for Senator Rubio," said Rob Engstrom, national political director at the U.S. Chamber of Commerce. "The party of Alan Grayson (and Bernie Sanders) is alive and well in Florida."

Rubio, in an interview with The Associated Press after making his announcement, declined to rule out another presidential run. "I'm done making these unequivocal statements about what I will or won't do," he said. But he added that he was running for re-election fully intending to try to come back to the Senate to make it a more productive place -- and argued that if he were planning to make another presidential bid, a tough and expensive Senate race now wouldn't be the best move.

"If the last political office I hold is to be a U.S. Senator from Florida and I'm able to do some really meaningful things, I'm at peace with that. I'll be pleased with that," Rubio said.