Senator Sherrod Brown, the populist Ohio Democrat known for extolling the “dignity of work,” announced Thursday that he would not seek the Democratic nomination for president, ceding the primary field to other left-leaning candidates with broader national appeal.

His exit comes as better-known Democrats like former Vice President Joseph R. Biden Jr. prepare to announce their own plans for 2020, and as the field itself has swelled to 14 candidates, including fellow liberal senators like Bernie Sanders and Elizabeth Warren.

Allies of Mr. Brown said on Thursday that his decision to forgo a 2020 bid had nothing to do with the signs that Mr. Biden is preparing to enter the race. Rather, they said, Mr. Brown ultimately resolved that he was content in the Senate and lacked the consuming drive to become president. And there were obvious practical obstacles to the 2020 nomination for a 66-year-old white man with a mix of liberal and moderate views and a relatively modest donor base. Indeed, for Mr. Brown, the popularity and fund-raising strength of Mr. Sanders, a fellow populist, were obstacles that were at least as big as any Mr. Biden might present.

[Why are so many Democrats bowing out of a 2020 campaign this week? Here are four reasons.]

Speaking to reporters between votes in the Capitol on Thursday, Mr. Brown quickly noted that he “didn’t have a lifelong ambition to be president.” Though he argued that there was a “path for me,” highlighting his progressive credentials, he said he felt tugged to remain in the Senate, a job he relishes more than some of his colleagues who covet the presidency.