[Who’s in, who’s out and who’s thinking about it. Check out our presidential candidate tracker.]

Long a progressive darling, Mr. Merkley rose to national prominence as the only senator to endorse Senator Bernie Sanders over Hillary Clinton in the 2016 presidential race. After Mr. Sanders lost, Mr. Merkley saw an opportunity to claim some, if not all, of the Vermont senator’s coveted base of support.

But he would have been far from the only candidate fighting for what’s become known as the “Sanders wing” of the Democratic Party. Senators Elizabeth Warren, Kamala Harris and Cory Booker have all staked a claim to a share of the liberal base.

“To win these battles we need both strong leadership in the Oval Office and strong leadership in the Senate,” said Mr. Merkley. “I believe that there are Democrats now in the presidential race who are speaking to the importance of tackling the big challenges that we face.”

During the first week of Mr. Sanders’s campaign for the 2020 nomination, aides announced that he’d raised $10 million, underscoring his still-strong hold over liberals in the party. Even before Mr. Sanders entered the race, Mr. Merkley had already raised doubts about whether he could build the kind of operation needed to run successfully in such a crowded primary field.

“You would really have to build a vast operation to be competitive given the design of our primary system,” he told Oregon Public Broadcasting this year, “and that would require an all-out effort. And so it’s balancing that effort against putting all my efforts in through the Senate.”