In a major reversal highlighting Republican fear over losing the Senate majority and his own plans for another presidential run, Marco Rubio on Wednesday broke a longstanding promise not to seek re-election, becoming an instant favorite but facing the challenge of running in a year featuring Donald Trump."Truth is, I never claimed to be perfect," the 45-year-old Rubio said in an interview with the Tampa Bay Times, framing the flip-flop as a way to serve Florida and keep checks and balances on the "excesses of a president.""Control of the Senate could very well come down to what happens in Florida," Rubio said, citing the fate of Supreme Court appointments, the "disastrous" Iran nuclear accord and Democrats' plans to shutter Guantanamo Bay."No matter who is elected the next president," he said, "there is reason to worry."Previous coverage: Rise and Stall: The political trajectory of Marco RubioRubio's move forced three Republicans from the Senate race, leaving two little-known political outsiders who have characterized Rubio as a career politician desperate to maintain power.Likewise, Democrats blasted Rubio as an opportunist and trotted out numerous negative comments he made about the Senate and attacked his "chronic absenteeism" while he ran for president."Marco Rubio abandoned his constituents, and now he's treating them like a consolation prize," said U.S. Rep. Patrick Murphy of Jupiter, the Democratic establishment favorite in the race.Though Rubio foreshadowed the decision for weeks, he insisted it came after the Orlando massacre and that it was made not in Washington but "back in West Miami" where he lives with his wife and four children.He was careful not to attribute the decision to Orlando but cited encouragement from his friend, Lt. Gov. Carlos Lopez-Cantera, who said on the day of the attack he would drop out of the race for Rubio. He followed through Wednesday."What happened in Orlando is horrifying and I think we need to do everything we can to make sure that never happens again, given the threat we face with radical terror in America," Rubio said. "But this goes beyond that. It's about the role the Senate can play."Further political cover arrived from national Republicans who choreographed an aggressive recruitment campaign, worried the current Senate field in Florida was too weak. A Quinnipiac University poll released Wednesday confirmed that concern, showing Democrats winning the seat unless Rubio runs.Democrats need to net as few as four seats nationally to take over the Senate. Rubio is no sure thing, but he makes a takeover more difficult. Though his presidential campaign coffers are exhausted (he could have transferred unused funds) he will be flooded with money from Washington and has a national donor base. He enjoys far more name recognition than any of his opponents. Endorsements rolled in the Club for Growth and Senate Leadership Fund, which is tied to Senate Majority Leader Mitch McConnell, and Sen. Ted Cruz. Former Gov. Jeb Bush said he'd gladly back Rubio "if he asks."Behind accolades for being a team player is the incessant ambition of a figure who wants to run for president again, as early as 2020 if Hillary Clinton wins. Rubio argued Wednesday it would be smarter not to run for Senate right after a presidential run. But the Senate would keep him in the limelight far more than the private sector, providing a platform to propose policy ideas and further develop foreign policy credentials he has made the focus of his time so far in Washington.He would not rule out running for president in 2020 or say whether he would serve all six years of a Senate term, if elected. "I'm done making unequivocal statements about anything," he said, adding he was "fully committed to coming back and making my mark in the Senate."PolitiFact Florida: Marco Rubio flip-flops on running for SenateAs a presidential contender over the last year, Rubio repeatedly said he would not seek re-election, dismissing the Senate as ineffective and out of touch."It's frustrating to watch every day nothing happen, no matter who's in charge, who you elect, who you give the majority to — nothing happens," Rubio said in New Hampshire in November. Another time he declared, "You're not going to fix America with senators and congressmen."In May, he tweeted: "I have only said like 10000 times I will be a private citizen in January."Now he'll have to confront those words in a state that handed him an embarrassing loss in the March 15 presidential primary. His candidacy was rejected by all of Florida's 67 counties except hometown Miami-Dade."Being in the U.S. Senate, working here, can be very frustrating, and that's true. I still believe that," he said Wednesday. But Rubio said he felt too much is at stake.He hinted at the possibility of a reversal almost immediately after dropping out of the race for president. He began traveling the state extensively, touching on issues ranging from slum apartments in Jacksonville to hurricane preparedness.In numerous interviews he argued that he never "hated" the Senate. He made sure not to miss votes. Then top Republicans in Washington mounted the campaign to draft him into the race.On May 27, Rubio cracked the window, saying "maybe" he would run if Lopez-Cantera were not in the race. On the day the nation woke up to the news that a gunman in Orlando had killed 49 people and wounded 53, Lopez-Cantera, reportedly told Rubio he would step aside. Still, Lopez-Cantera had run a lackluster campaign, fanning speculation about coordination the two men deny.Sensing what was coming, Rep. David Jolly, R-Indian Shores, dropped out of the Senate race last week to run for the House again. Rep. Ron DeSantis, R-Ponte Vedra Beach, withdrew Wednesday to do the same.That leaves Rubio to contend in a primary against Bradenton developer Carlos Beruff and former CIA agent Todd Wilcox, both of whom have said they would not be bullied by a Washington power play that conjures the bench-clearing Republicans attempted in 2010.Then-Republican Gov. Charlie Crist was the favorite six years ago but Rubio stuck in the race, seizing the nascent tea party movement and becoming a national star on his way to running for president.In 2016, Rubio faces a different dynamic. The tea party has faded and he has not been a conservative favorite for years, largely over his role in writing the Senate's 2013 comprehensive immigration bill.Beruff, assuming the antiestablishment role Rubio once claimed, has already spent $4 million of his own money on TV ads and has promised to drop as much as $20 million. He's trying to define Rubio as Crist-like, doing whatever it takes to remain in power."Mr. Rubio has become part of the establishment," Beruff told about 30 people during a campaign stop Wednesday in Panama City.The strains of Rubio's focus away from Florida were evident. Attorney William G. Harrison Jr., who said he supported Rubio in 2010, said Rubio had fallen out of touch."Sure we've seen him on TV during the presidential campaign. But around here people know if you have a problem you call Bill Nelson's office and they will take care of it," Harrison said. "(Rubio's) a good guy, but you just don't know who he is. We don't have a connection with him."If Rubio wins the Aug. 30 primary, the general election will be more challenging. Democrats turn out the vote in a presidential year and will be especially motivated with Trump on the ballot. Hispanics alone are registering just to oppose him. A Quinnipiac poll this week showed Clinton expanding what had been a slim lead over Trump in Florida."Marco's going to have a tough time because Hillary is going to sweep Florida," said Democratic Sen. Bill Nelson.Rubio seems well aware of the dynamic and sought distance from Trump in an array of interviews Wednesday."The prospects of Donald winning the presidency is worrisome, too," Rubio told the Times. "It's no secret that I have significant disagreements with him. His positions on many key issues are still unknown. Obviously many of the statements, especially about women and minorities are things that I find not just offensive but unacceptable."He does not plan to campaign with Trump.Times/Herald staff writer Jeremy Wallace contributed to this report.