Randi Weingarten, the longtime leader of one of the nation’s most influential labor unions and a major voice in Democratic politics, has resigned from the Democratic National Committee. She pointed to disagreements with the party’s new chairman, Ken Martin.

Ms. Weingarten, the president of the American Federation of Teachers, had supported Mr. Martin’s rival in the chairmanship race early this year, Ben Wikler, the chairman of the Wisconsin Democratic Party. Mr. Martin subsequently removed Ms. Weingarten from the party’s Rules and Bylaws Committee, a powerful body that sets the calendar and process for the Democratic Party’s presidential nominating process.

In her resignation letter, dated June 5 and obtained on Sunday evening, Ms. Weingarten wrote that she would decline Mr. Martin’s offer to reappoint her to the broader national committee, on which she has served since 2002. She had been on the Rules and Bylaws committee since 2009.

“While I am proud to be a Democrat, I appear to be out of step with the leadership you are forging, and I do not want to be the one who keeps questioning why we are not enlarging our tent and actively trying to engage more and more of our communities,” Ms. Weingarten wrote in her resignation letter to Mr. Martin.

Ms. Weingarten is an influential figure in the Democratic Party and the leader of a union that counts 1.8 million members.

Mr. Martin has recently faced scrutiny and criticism from within the party. His leadership was openly challenged by David Hogg, a party vice chairman who announced he would fund primary challenges to sitting Democrats — an action long considered out of bounds for top party officials.

Mr. Hogg announced last week that he would not seek to retain his post after the party voted to redo the vice chair election, after it had been challenged on an unrelated technicality.

On Friday, during an appearance at the Center for American Progress in Washington, Gov. Tim Walz of Minnesota, a longtime Martin ally, said he still had confidence in him but regretted the public squabbling.

“I certainly wished we wouldn’t have dirty laundry in public, but you know the personalities, things happen,” said Mr. Walz, who endorsed both Mr. Martin and Mr. Hogg in the party elections this year. “I don’t think Ken’s focus has shifted one bit on this of expanding the party.”

Mr. Martin and a party spokeswoman did not immediately respond to messages about Ms. Weingarten’s resignation.