Alluding to Mr. Trump’s Oval Office address on Tuesday night, in which he demanded the construction of a border wall, Mr. Steyer said the president had “once again lied to the American people, repeatedly, for his own political skin.” He described Mr. Trump as having already committed numerous offenses warranting his removal from office, and warned Democrats that shying away from an impeachment battle would serve to “enable” the president.

Mr. Steyer, 61, left himself some wiggle room to change his mind on 2020, saying in prepared remarks that he had decided against running “at this time.” But his announcement ended — at least for now — the latest of several flirtations with seeking high office, which have also included abortive candidacies for the United States Senate and for governor of California. He considered running for president in 2016 before ultimately endorsing Hillary Clinton.

In some respects, the Democratic primary landscape appeared inviting for a candidate like Mr. Steyer, with his sterling credentials as a Trump antagonist and a virtually bottomless well of money to spend on advertising. He has been one of the Democratic Party’s most prolific donors over the last few elections, eclipsed in 2018 only by Michael R. Bloomberg, the former New York City mayor who is considering a presidential campaign of his own.

Anticipating a likely race, Mr. Steyer had settled on a close adviser, Heather Hargreaves, to serve as his eventual campaign manager. He had conducted research into his own political vulnerabilities, in anticipation of attacks from other Democrats in a rowdy primary, and had mapped out how to reorganize his advocacy groups to comply with the fund-raising regulations that apply to presidential candidates.

Mr. Steyer also recently retained a new senior adviser, Doug Rubin, who previously advised former Gov. Deval Patrick of Massachusetts on a possible 2020 run. Mr. Patrick announced last month that he had closed the door on a presidential candidacy.