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.

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.

Senate Democrats are scheduled to vote behind closed doors on Dec. 1 to determine who will be their leader when their party takes the Senate majority on Jan. 4.

“I think there is a big appetite for change within the Democratic Caucus,” Mr. Van Hollen said in an interview before the announcement. “I think people want to have a system of much more meaningful input, and that’s going to be even more important now that we have this large majority.”

His announcement came as Mr. Schumer and his allies reiterated that the New York Democrat had commitments of support from well over the majority of Senate Democrats needed to re-elect him as leader.

In a statement, Mr. Schumer said that he welcomed “a discussion about the future of our caucus and the important work ahead.”

“I’m proud of what we’ve accomplished together — 55 seats and counting is an amazing result — and I have the votes to continue serving as Democratic leader,” he said. “I look forward to leading our new Democratic majority and delivering for the American people.”

Mr. Schumer oversaw the Senate Democratic campaign effort that has flipped at least eight Senate seats, with the Democrat Scott Colom on Wednesday called as the winner in the Mississippi Senate race. The Alaska Senate race result has not been called, but the Democrat Mary Peltola is slightly ahead there.

Despite that success, some in the ranks of Senate Democrats have called for a broad discussion of party leadership going forward and a debate over the agenda the party should pursue.

Other Democrats say that the leadership fight is distracting from the stunning victory Democrats recorded on Nov. 3 and hurting their preparation for January.

“We were at 47-53 and now we are at 55, 56,” said Senator John Fetterman, Democrat of Pennsylvania and a strong supporter of Mr. Schumer. He added: “I don’t understand why that’s a conversation.”

Other Democrats have noted the contrast of Senate Republicans returning their leadership team without a challenge after losing at least 8 seats while Democrats are embroiled in a showdown despite their win.

Mr. Van Hollen, 67, is the child of a diplomat and was elected to the House in 2002 after entering the race as an underdog in a very competitive primary while serving in the Maryland legislature.

In the House, he became a close ally to Speaker Nancy Pelosi and a leading voice on budget issues. In 2008, he led the House campaign effort as Democrats expanded their majority when Barack Obama was elected president and also led the campaign team in 2010.

Mr. Van Hollen was seen as a potential future speaker in the House, but that prospect seemed far-off given Ms. Pelosi’s hold on the office. When Barbara Mikulski of Baltimore was set to retire from the Senate in 2016, he sought her seat and won. Given his experience, Senate Democrats immediately put him in charge of their campaign organization.

In the Senate, Mr. Van Hollen has focused on budget issues and foreign policy. He has been an outspoken proponent of cutting off money for the war against Iran.

Progressive activists have urged that Mr. Schumer, 75, face a challenge to remaining in the position. Donors and organizations including MoveOn, Indivisible and the Working Families Party plan to back and mobilize members on behalf of Mr. Van Hollen. They are threatening to back primary challenges against any Democratic senator up for re-election in 2028 who does not come out in support of him.

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

Mr. Schumer and his allies have been adamant that he has more than enough votes to hold the position. But secret ballot leadership votes can be unpredictable. Some senators have refrained from going public with their support to avoid coming under pressure from those urging that Mr. Schumer be deposed.

Mr. Van Hollen says he was not deterred by Mr. Schumer’s claim on the votes to prevail.

“Circumstances change,” he said. “It is one thing to have someone’s support when you’re the only choice. I think a little democracy in the Senate Democratic Caucus is a good thing.”

Lisa Lerer and Reid J. Epstein contributed reporting.