By Patricia Zengerle and Kouichi Shirayanagi

WASHINGTON (Reuters) - Former Republican presidential candidate Marco Rubio will seek re-election to his U.S. Senate seat, he announced on Wednesday, reversing a pledge not to run and saying the prospect of a Donald Trump presidency worries him.

The first-term Florida senator said the Senate can be frustrating, but that it is also a place where policy advances can be made and one that can counteract the power of the White House.

"But as we begin the next chapter in the history of our nation, there's another role for the Senate that could end up being its most important in the years to come: The Constitutional power to act as a check and balance on the excesses of a president," Rubio said in a statement.

The 45-year-old lawmaker, a proven fund-raiser who is well known nationally, had been under intense pressure to run, especially given the cost of contesting an important election swing-state where campaign advertising is expensive.

Republicans are more likely to retain his seat if he is a candidate in the Nov. 8 general election, as they face a tough fight to preserve their majority in the Senate. He could also rally Republican voters to head to the polls in a state that is critical in the presidential race.

President Barack Obama, a Democrat, took Florida by just a one percentage-point margin in 2012 over Republican Mitt Romney.

This year, Republicans must defend 24 of the 54 seats they now hold in the 100-member Senate, compared with just 10 the Democrats have to fight to retain. Senate Majority Leader Mitch McConnell has said it would be a "terrific outcome" if Rubio decided to run.

Rubio had harsh words for Hillary Clinton, the presumptive Democratic presidential nominee, predicting her election would mean four more years of what he termed Obama's "failed" policies.

TRUMP 'WORRISOME'

But he said the prospect of Republican presidential candidate Donald Trump winning the White House was also "worrisome," citing Trump's statements about women and minorities and noting that many of the likely Republican nominee's positions on important issues are unknown.

"If he is elected, we will need Senators willing to encourage him in the right direction, and if necessary, stand up to him," Rubio's statement said.

Still, he pledged to vote for Trump, a real estate mogul and political neophyte who has shaken the Republican establishment.

"It's not the choice I wanted ... and it's not the choice a lot of people are comfortable with, but it's the choice we have," he told CNN in an interview.

His announcement did not mention the massacre last week at a gay nightclub in Orlando, Florida, but he had said in recent days he was reconsidering his decision not to run because of his concern about serious events in the country, including the mass shooting.

The decision reversed a campaign pledge Rubio made a year ago to retire from the Senate if his presidential bid failed. He ended his bid for the nomination in March after losing the primary in his home state to Trump following a harsh war of words.

The Cook Political Report said Rubio's announcement "breathed new life" into Republican chances of holding onto the seat but still rated the race as a toss-up, citing his poor Senate attendance record and the senator's remarks during his presidential campaign that he was "frustrated" with the upper chamber.

Republican U.S. Representative David Jolly, who had been seeking the Senate seat, opened the door for Rubio to run when he stepped aside last week.

After Wednesday's announcement, Florida Lieutenant Governor Carlos Lopez Cantera, a friend of Rubio, and Republican U.S. Representative Ron DeSantis also scrapped plans to seek the seat. Other Republicans, however, have not announced plans to quit ahead of Florida's Aug. 30 primary.

U.S. Representative Patrick Murphy, a leading Democratic challenger, accused Rubio of first abandoning his constituents, and now "treating them like a consolation prize."

(Reporting by Patricia Zengerle and Kouichi Shirayanagi; Writing by Patricia Zengerle; Editing by Tim Ahmann and Jonathan Oatis)