(CNN) A group of Democrats determined to unseat Nancy Pelosi will circulate a letter this week, collecting signatures from incumbents and incoming freshmen who oppose the longtime Democratic leader as she seeks another speakership, according to three sources familiar with their letter.
Meanwhile, 14 different Democrats signed a letter Monday defending Pelosi and calling to keep in place caucus rules that would make it easier for her to clinch the nomination.
The dueling letters come the same week that members return to Washington for the first time since the midterm elections, in which Democrats won control of the House for the first time in eight years, while Republicans maintained their majority in the Senate. Incoming freshmen will also be in town this week for orientation.
The group of Pelosi detractors has been working to recruit a challenger to the California Democrat, the heavy favorite for speaker, but no one has stepped forward. They've also been encouraging incoming freshmen who pledged to oppose Pelosi to follow through with their campaign promise, offering support as they enter a divided caucus and face pressure to change their minds.
With the new letter, they hope to send a clear message to the caucus that Pelosi has a difficult path ahead to get 218 votes, the number traditionally needed to win the speakership on the floor. According to one source familiar with the letter, they expect to have more than 20 signatures, but it's unclear yet when they'll formally send the letter to the caucus.
According to a CNN tally, 13 incumbents and 10 incoming freshmen have publicly stated they will oppose Pelosi for speaker, though not all have stated whether that will be in the initial caucus vote on Nov. 28 or also on the House floor in January.
Pelosi aides are confident she'll have enough support in the end for the floor vote. Depending on the final majority number for Democrats, Pelosi could win the speakership with fewer than 218 votes if a certain number of people vote "present," which lowers the threshold.
CNN has so far projected that Democrats have a 225-seat majority, with 10 races uncalled.
Pelosi allies also expect that some of the members who vote against her in the caucus election will come around and vote for her on the floor. In 2016, 63 members voted against her in the caucus, but almost all of them supported her on the full House vote.
Supporters of Pelosi are also lobbying incoming freshmen, urging the caucus to stay united for the floor vote.
The group of 14 Pelosi supporters on Monday zeroed in on a push by the detractors to change caucus rules and make it more difficult for her to lock up the nomination during the caucus election later this month.
As it stands, the nomination can be won with only a majority of the caucus. Pelosi opponents want to raise that threshold so the nominee comes out of the caucus election with a much higher threshold of support. They previously suggested raising it to 218 -- the same number needed for the floor -- but are open to a lower number. They gathered enough signatures last week to trigger a discussion about it in the next caucus meeting, set for Wednesday and Thursday.
Pelosi supporters are publicly pushing back on changing the rules. In the letter they sent to colleagues Monday, they argued that raising the threshold to as high as 218 in the caucus could allow a small group of members to block the will of the vast majority.
"I don't know of any political party on earth that operates on that principle," Rep. Jamie Raskin of Maryland told CNN. "It simply wouldn't work."
The letter encouraged anyone who wants to run for leadership to run in the caucus elections, but also called for Democrats to unite around the nominee on the floor.
"Once we leave that room, we are Democrats and we've got to stick together in order to provide direction for the country," Raskin said.
Along with Raskin, signatories of that letter were: Rep. Don Beyer of Virginia, Andre Carson of Indiana, Val Demings of Florida, Ted Deutch of Florida, Lloyd Doggett of Texas, Dwight Evans of Pennsylvania, Lois Frankel of Florida, John Garamendi of California, Jimmy Gomez of California, Doris Matsui of California, Betty McCollum of Maryland, Jan Schakowsky of Illinois and Eric Swalwell of California.