Senator Chris Van Hollen of Maryland announced on Thursday that he would challenge Senator Chuck Schumer of New York for majority leader, setting up a clash among Democrats just as they were celebrating their new majority and planning for a takeover of Congress in January.

The move by Mr. Van Hollen, a progressive Democrat elected to the Senate in 2016 after seven terms in the House representing the Washington, D.C., suburbs, comes as some Democratic senators and activists have called for an alternative to Mr. Schumer, a moderate whom they regard as too tied to the party establishment to lead a newly energized party forward in the next Congress.

Making his announcement in an interview on MS Now on Thursday evening, Mr. Van Hollen said his challenge to Mr. Schumer was “not about any one individual.” Instead, he said, it was to ensure that Democrats answered “the call of the American people” in the midterm elections, which he said was to “bring down costs” and end the war against Iran.

He said he had spoken to a large number of Democratic senators, including the incoming members, and did not believe that Mr. Schumer had a lock on the race, saying he had “more than 10” of the 55 Democratic senators and senators-elect behind him.

“Am I the underdog?” he said. “Yes.”

Mr. Van Hollen, who is also said to be weighing a run for president in 2028, had emerged as the favorite to challenge Mr. Schumer for the post and had expressed increasing willingness to do so in recent days.

Senator Bernie Sanders, an independent from Vermont who aligns with Democrats, said this week that he would oppose Mr. Schumer and had encouraged Mr. Van Hollen to become a candidate.