Democrats are poised to nominate Nancy Pelosi for House speaker in a a secret, closed-door ballot on Wednesday that will serve as a critical test of her political power amid an effort to topple the senior party leader.

Despite a vocal resistance, Pelosi is running unopposed for the post and is expected to easily win a majority of her caucus. But the ballot is a preliminary step and does not assure Pelosi’s return to speakership. That will be settled by a floor vote when the new Congress convenes in January – and her Democratic opponents are confident Wednesday’s caucus will reveal that Pelosi does not have the support within the party to win the majority of House members needed to secure the speakership.

The intra-party battle to choose a new House speaker has pitted the powerful leader and first female speaker against a group of newcomers and incumbents demanding a fresh face, and other detractors seeking to extract concessions in exchange for their support.

Pelosi, 78, known in Washington as a master deal-maker, has moved aggressively to counter the mini-rebellions, holding dozens of one-on-one meetings in her office and quietly horse-trading with would-be opponents. She has especially courted new members of the Democratic class, who represent the party’s largest margin of victory in midterm House races since the Watergate scandal in 1974.

On the eve of the crucial caucus vote, nearly two dozen newly elected Democrats declared their support for Pelosi. A letter signed by 20 Democratic freshmen released on Tuesday hailed Pelosi for her work to pass the Affordable Care Act during her last turn as speaker in 2010.

“She delivered health care to millions of Americans, and voters put us back into the majority largely because of our promises to protect and expand people’s access to that care,” they wrote in the letter, that ended with a call for Democrats to “unite” and “move forward”.

In a separate statement on Tuesday, Jahana Hayes, an incoming freshman Democrat from Connecticut who had called for leadership change, said that she was “not willing to make change just for the sake of making change”.

“You can’t beat somebody with nobody, and the fact is that voting against Pelosi on the floor is a vote for Republicans,” she said.

Pelosi has expressed unflinching confidence that she has the roughly 218 votes to succeed, assuming all Republicans vote against her. But her opponents insist she does not yet have the votes to win with Democratic support alone, and that her only option is to negotiate with her opponents.

“Pelosi wants to boil this down to a personal argument, but this is so much bigger than her,” Congressman Seth Moulton, a leader of the anti-Pelosi resistance, said in a statement this week. “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.”

Pelosi has shown no willingness to abandon her longtime deputies, congressman Steny Hoyer, 79, of Maryland, and congressman James Clyburn, 78, of South Carolina, who are running unopposed for the No 2 and No 3 leadership positions. In a show of strength on Tuesday, Hoyer released a letter endorsing his candidacy that was signed by more than three-quarters of House Democrats. His broad support from the caucus has hampered an attempt by some of Pelosi’s opponents to seize power by mounting challenges for the other top posts if the effort fails to end her nearly 16-year reign atop the Democratic party.

A source close to the effort to block Pelosi said that she could assuage members’ concerns by offering a clearly defined exit strategy, such as an assurance to serve for only one term. But the California Democrat has so far been unwilling to offer such a commitment. Pelosi’s supporters say she has resisted calls to specify a departure date because doing so would make her a lame-duck leader.

Meanwhile, the demands of a separate group of moderate Democrats add another dimension to the leadership fight. A group of nine Democrats from the bipartisan House Problem Solvers Caucus has threatened to withhold votes for Pelosi unless she agrees to a series of rule changes that they say would improve how the House operates.

Though Pelosi has yet to entirely flatten the obstacles in her path, it appeared increasingly likely in the hours before Wednesday’s ballot that her leadership team would emerge intact.

With Democrats prepared to re-elect familiar faces to the top posts, the intrigue has turned to the races for lower-level leadership posts, such as the competitive contests for the chairmanship of the Democratic caucus and the leader of the House Democratic campaign arm.

As part of her campaign to quell calls for new leadership, Pelosi promised to be a “transitional” speaker and to build a bridge for a younger generation of party leaders. And when Pelosi and her deputies eventually step down, the Democrats holding those jobs are well-positioned to be next in line.