"I changed my mind," Rubio said frankly in an interview with CNN Wednesday. "I honestly believe that no matter who is elected president we're going to need a Senate full of people willing to act as a check and balance on the excesses of the next president, and I think given the state of this race we're going to have a lot of that over the next few years."
Rubio's announcement marks a 180-degree turn from where he was even just a month ago, when he insisted he would give up his Senate seat at the end of this term. He even used the fact he wasn't running as an excuse for missing a large number of votes during his unsuccessful bid for the Republican presidential nomination.
Speaking with CNN, Rubio defended his voting record, saying he had helped "thousands of people in Florida" though his office.
But he wouldn't commit to serving a full six-year term if re-elected.
"What I'm not going to do any more are these unequivocal pronunciations," he said, avoiding a question about whether he planned to run for president again in 2020.
"If I were running for president in 2020, jumping into this race is not the decision most people would advise," Rubio said. "I'm coming back to be a U.S. senator and give my time and energy and have a Senate that can act as a check and balance on whoever is elected president."
In an email to supporters Wednesday, Rubio said he's aware his opponents will use his decision "to score political points against me."
"Have at it," Rubio wrote. "Because I have never claimed to be perfect, or to have all the answers."
The decision follows a month of heavy lobbying from top Republicans and soul-searching on the part of Rubio after the Orlando attack.
Many Republican colleagues encouraged him to run, believing he's the party's best chance to keep the seat. His former presidential primary opponent Sen. Ted Cruz announced his support shortly after news broke of Rubio's decision.
"Marco is a friend and has been an ally in many battles we have fought together in the Senate," Cruz said. "I'm glad to support him in his bid for re-election. Marco is a tremendous communicator and a powerful voice for the American Dream.
But not every Republican in the highly competitive Florida Senate primary was happy about the decision. Rubio still faces two Republican primary rivals: Businessmen Carlos Beruff and Todd Wilcox.
Another challenger Rep. Ron DeSantis said in a statement that Rubio's entry "changes the contours" of the race, and that he could "best advance the cause" by running for reelection in the House.
Leading Democrats have already painted a bright target on Rubio -- Montana Democrat Jon Tester, leader of the Senate Democrats' campaign arm, said last week that beating Rubio could potentially kill any chances he has for the White House in 2020.
And the vicious Republican primary -- which Rubio exited only a few months ago -- is sure to be milled for attacks against Rubio.
Rep. Patrick Murphy, a favorite among establishment Democrats for the seat, quickly blasted Rubio on Twitter for missing numerous key Senate votes and intelligence briefings to campaign for the White House.
"Marco Rubio abandoned Florida. Unlike Rubio, I love working hard every single day for the people of Florida. #flsen," Murphy tweeted, shortly after news leaked that Rubio was running again.