But Mr. Bauman was always a contentious figure in the party — viewed as too moderate in his politics and conventional in his tactics. He won the job by just 60 votes out of 3,000 cast. As a result, he had little political capital to weather this kind of storm.

The calls by Mr. Newsom — the lieutenant governor who is soon to be the state’s highest-ranking Democratic elected official — made Mr. Bauman’s position politically untenable.

“I have made the realization that in order for those to whom I may have caused pain and who need to heal, for my own health, and in the best interest of the Party that I love and to which I have dedicated myself for more than 25 years, it is in everyone’s best interest for me to resign my position as chair of the California Democratic Party,” Mr. Bauman said.

Mr. Bauman was known as a tough-talking, old-school kind of political leader, a style that he took with him from the Bronx when he came west. His departure opens up the way for a leadership battle for the party — which is already facing tension between moderates and activists — as it prepares for the 2020 presidential contest.

Among possible successors are Kimberly Ellis, the San Francisco Democrat who lost to Mr. Bauman. Representative Ro Khanna, a Silicon Valley Democrat who called on Mr. Bauman to step down as soon as the charges became public, has suggested Michele Dauber, a Stanford University law professor, as a successor.