Marco Rubio will run for re-election for his Florida Senate seat, reversing his previous decision to leave Capitol Hill.
Rubio, who ended his White House bid earlier this year, had long insisted that he would be a "private citizen" after his current Senate term expires. But he came under intense pressure from his own party to seek reelection amid concerns that Republicans could lose their majority in the United States Senate.
Acknowledging in a statement that "in politics, admitting you've changed your mind is not something most people like to do," Rubio said that maintaining GOP dominance in the Senate was a key reason for his reversal.
"Control of the Senate may very well come down to the race in Florida. That means the future of the Supreme Court will be determined by the Florida Senate seat," he said. "It means the future of the disastrous Iran nuclear deal will be determined by the Florida Senate seat. It means the direction of our country's fiscal and economic policies will be determined by this Senate seat."
Rubio, who aggressively opposed the nomination of GOP presumptive nominee Donald Trump during his own presidential run, also said "there is reason for worry" whether Trump or Hillary Clinton is elected president.
If Trump is elected, he said, "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."
With a national profile and a deep fundraising network, Rubio was widely seen as the party's best standard-bearer in the state for what's sure to be an expensive and hard-fought general election race.
The deadline for Rubio to file for reelection is Friday, and the Senate primary will be held August 30.
Rubio indicated last week that he was reconsidering his pledge, suggesting that the massacre at a club in his home state had prompted him to think about his role as a public servant.
An aide confirmed the NBC News that the shooting was a motivator in his decision-making process.
He reiterated some of those concerns in an interview with the Miami Herald Wednesday.
"I think that the point that really drove me to change my mind is that as we enter this kind of new chapter in our history here is, there's another role the Senate plays that I think can be really important in the years to come," Rubio told the Herald. "And that's the power given to it in the Constitution to act as a check and balance on the excess of the president. It's even more important given the fact that control of the Senate could very well come down to what happens in the Florida race."
Rubio had also expressed reluctance to enter the race because a close friend, Lt. Gov. Carlos Lopez-Cantera, was already seeking the Senate seat. But Lopez-Cantera told supporters last week that he would drop his efforts if Rubio decided to jump in.
Another GOP challenger, Carlos Beruff, on Wednesday called Rubio "Washington's candidate," saying "This isn't Marco Rubio's seat; this is Florida's seat. The power brokers in Washington think they can control this race."
A Quinnipiac poll out Wednesday showed that Rubio leads Democratic Rep. Patrick Murphy 47 percent to 40 percent. Murphy faces his own primary against Rep. Alan Grayson in August.
In a statement, Murphy said ""Marco Rubio abandoned his constituents, and now he's treating them like a consolation prize."