The battle among Democrats to pick a new House speaker is turning into trench warfare, with Rep. Nancy Pelosi and her opponents trading gains in a clash that could last for weeks.

Rep.-elect Gil Cisneros (Calif.) said Monday that he was joining 15 other Democrats who are calling for fresh leadership, saying that “it’s time for a new generation to rise” — and that Pelosi, the House minority leader, is not part of the equation.

Cisneros’s opposition came hours after one of the leaders of the resistance, Rep. Seth Moulton (D-Mass.), signaled he was open to negotiations about changes to Pelosi’s leadership team — a retreat from his previous demand that Pelosi step aside.

Meanwhile, Pelosi (D-Calif.) faced another challenge from a group of nine centrist Democrats from the bipartisan Problem Solvers Caucus who renewed a vow to oppose her unless she agrees to rule changes aimed at easing cross-aisle legislating.

One opponent, Rep. Stephen F. Lynch (D-Mass.), said he could envision a scenario in which he votes for Pelosi in January.

The flurry of developments underscored two running themes in House Democratic politics following this month’s sweeping gains in the midterm elections: Pelosi has deep and wide support among her colleagues and the party at large, but she still faces doubts from incoming freshmen who faced Pelosi-themed attacks in the campaign as well as disgruntled incumbents unwilling to pass up the opportunity to install new leaders.

Democrats are scheduled to meet behind closed doors Wednesday to nominate a speaker and choose party leaders. Pelosi is expected to easily prevail, but the math becomes more difficult Jan. 3, when Republicans also get a say on who succeeds retiring Speaker Paul D. Ryan (R-Wis.). To win, a speaker candidate must garner a majority of all members who cast votes.

After pushing off questions about Minority Leader Nancy Pelosi until after the election, many non-incumbent Democrats are still dodging questions about her.

Cisneros’s opposition gave a jolt of energy to the anti-Pelosi dissidents, who had seen Pelosi’s lobbying pluck off several of the rebels. Last week, Reps. Marcia L. Fudge (D-Ohio) and Brian Higgins (D-N.Y.) backed Pelosi after earlier signaling their dissatisfaction.

“I talked with Leader Pelosi last week and she made the case why it’s important for her to be speaker,” Cisneros said in an interview. “It was a cordial conversation. I thanked her for her years of service and said she’s an icon for Democrats.”

Cisneros, during a long and expensive battle for the seat that has been held by retiring Rep. Edward R. Royce (R), repeatedly called for new party leadership without specifically saying he would oppose Pelosi as leader.

At the same time, there were signs that Pelosi’s campaign has started to wear down her opposition.

“If it becomes a choice between a Republican and Nancy Pelosi, I’ll obviously support Nancy Pelosi,” Lynch told WCVB, the ABC affiliate in Boston, in a TV interview Sunday.

Moulton said in a statement that the leadership contest is “so much bigger than her.”

“It’s about the entire, stagnant three-person leadership team and having a serious conversation about promoting leaders who reflect the future of our caucus,” he said.

Pelosi, 78, has given no indication that she is open to talks with Moulton about a deal or that she would waver in her support for her longtime deputies, Maryland Rep. Steny H. Hoyer, 79, and South Carolina Rep. James E. Clyburn, 78, who are in line to hold the No. 2 and No. 3 posts in the House next year.

A Pelosi aide who was not authorized to comment publicly said Monday that she had not received a request for a meeting.

Rather than negotiate with her opponents as a bloc, Pelosi has been focused on convincing members by focusing on their unique circumstances. And she has firmly resisted another suggestion from her critics: that she announce a certain date for her departure, making way for a fresher face.

“That undermines her ability to negotiate,” said Higgins, who broached the subject in his talks with Pelosi last week. “Other members are looking for something a little bit more specific, but I don’t think that’s realistic given the fact that the House needs all the leverage it can muster.”

The fresh demands from the Problem Solvers Caucus came after pushback from Rep.-elect Alexandria Ocasio-Cortez (D-N.Y.), a young progressive star of the incoming freshman class who in a holiday weekend tweet referred to “GOP-friendly rules” that “will hamstring healthcare efforts from the get-go.”

The nine Democrats in the caucus initially threatened to withhold their votes in a Nov. 13 letter. Now, with behind-the-scenes talks sputtering, they are renewing their demands.

The tiff stands as an early test of Pelosi’s ability to balance the wishes of more moderate lawmakers — “majority makers” who have been able to win swing and Republican-leaning districts — against an aggressive new crop of young progressives more interested in confrontation than compromise.

The Problem Solvers Caucus had been agitating for rules changes months before the election and released a letter in September, signed by Republicans and Democrats, warning both parties that they would withhold their votes unless the speaker met their demands.

Pelosi released a draft rules proposal last week that incorporated some of the caucus’s priorities but omitted several of the more substantial demands. Instead, the proposal floated the creation of a special committee to explore further changes.

According to aides familiar with the discussions, negotiations between Pelosi and Rep. Josh Gottheimer (D-N.J.), the group’s co-chairman, continued behind the scenes but reached an impasse over the holiday weekend.

In its statement Monday, the group doubled down on three demands: a “debate and timely vote” on any bill that is co-sponsored by three-fifths of House members; a guaranteed debate and vote on any amendment that wins at least 20 Democratic and 20 Republican co-sponsors; and a provision requiring committees to debate and vote on at least one bill from each of its members, provided that the measure is relevant to the panel’s jurisdiction and has at least one co-sponsor from the other party.

“The bottom line is this: We need real rules reform to get bipartisan legislation heard — not just more committees to study the problem,” the group said. “Bipartisan legislation with broad support deserves honest debate and a simple up-or-down vote.”

In her tweet, Ocasio-Cortez appeared to reject the premise that more bipartisanship is needed in the House.

“People sent us here to get things done, not ‘negotiate’ with an admin that jails children and guts people’s healthcare,” she wrote.

The Problem Solvers Caucus also is facing pushback from influential Democrats who don’t want to overly dilute the majority leadership’s power to set the House agenda just as their own party takes charge.

Rep. Jim McGovern (D-Mass.), the incoming Rules Committee chairman, said last week that some of the Problem Solvers’ proposals were praiseworthy but that others could make the House “chaotic” and hard to manage.

“What we’re not going to do is set up a situation where we allow this place to become the Senate,” he said.

The Pelosi aide said talks with the caucus are ongoing. Pelosi is set to meet with some of the group’s Democrats on Tuesday.

Although Pelosi has no declared Democratic opponent, some of her critics argue that an alternative will emerge if she does not secure enough votes to win. In the past, members of both parties have cast protest votes for candidates who were not seeking the speakership.

Other Democrats who have been undecided about their speaker votes have trickled out in support of Pelosi in recent days, including Reps.-elect Sharice Davids (Kan.) and Ayanna Pressley (Mass.)

John Wagner contributed to this report.