WASHINGTON, March 7 - Democratic Senator Sherrod Brown of Ohio said on Thursday he would not enter the 2020 US presidential race, but would work to help the party recapture the White House and battle Republican President Donald Trump's "phony populism."

Brown, who has just wrapped up a month-long tour of four early voting states as he explored a presidential bid, said he was confident the growing field of Democratic White House contenders would keep up his fight for workers and his message about "the dignity of work."

"We've seen candidates begin taking up the dignity of work fight, and we have seen voters across the country demanding it," Brown said in a statement. "It is how we beat Trump, and it is how we should govern."

Brown, a liberal with strong ties to the labor movement, had not registered significant support in most opinion polls of a growing Democratic field that includes better-known fellow senators such as Kamala Harris of California, Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts.

His decision not to run is certain to increase speculation that former Vice President Joe Biden, another Democrat with a history of appealing to working-class voters, is likely to enter the race. Former Congressman Beto O'Rourke of Texas is also expected to announce a decision soon.

Three other possible Democratic contenders - former New York Mayor Michael Bloomberg, former US Attorney General Eric Holder, and US Senator Jeff Merkley of Oregon - also announced this week that they would not run.

Reuters