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.

On Thursday night, Senator-elect Abdul El-Sayed of Michigan, who during his campaign called on Mr. Van Hollen to seek the post, greeted his announcement enthusiastically, saying he was “proud” to endorse him as majority leader.

“Last week, Michiganders rejected the failed status quo and the politics that too often leaves families behind,” Mr. El-Sayed said in a statement. “But we need to earn our mandate. That demands new leadership.”

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.

In a statement, Mr. Schumer said that he welcomed “a discussion about the future of our caucus and the important work ahead,” but had enough commitments of support from Senate Democrats to be re-elected as leader.

“I’m proud of what we’ve accomplished together — 55 seats and counting is an amazing result,” he said, adding: “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 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.”

On Thursday night, Senator Brian Schatz, Democrat of Hawaii, said he welcomed a conversation “about the path ahead,” but urged his colleagues not to draw out that process. He endorsed Mr. Schumer, whom he credited with leading the party to victory.

“We must move quickly to vote, unify, and get to work,” Mr. Schatz said.

Senator Angus King of Maine, the other independent who caucuses with Democrats, also threw his support behind Mr. Schumer in a statement, crediting him with engineering a historically successful election result.

“Chuck has outlined an aggressive plan for the new Democratic majority, which is directly responsive to the message we heard from the voters, and he has the experience and unique mastery of the Senate process to make it happen,” Mr. King said. “Why would you fire the manager when you just won the World Series?”

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 had 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.

On Thursday, he said Democrats’ Senate campaign arm should not take any funding from AIPAC, the pro-Israel lobbying group that many progressives have shunned in recent years.

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 are likely to back and mobilize members on behalf of Mr. Van Hollen, according to a person familiar with progressive groups’ strategy who spoke about it on the condition of anonymity. Some of the donors plan to threaten to back primary challenges against any Democratic senator running for re-election in 2028 who does not come out in support of him, the person said.

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.

“Circumstances change,” Mr. Van Hollen 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.