WASHINGTON — As Democrats battle to retake control of Congress in November, their leader — Nancy Pelosi — could also be facing a coming fight of her own.

Fifty Democrats running for the House say they won't support the California lawmaker for speaker, according to an NBC News survey of candidates and their public statements.

At least 42 of the party's nominees for House seats have declared they will not back Pelosi and nine incumbent Democratic lawmakers are on the record opposing her, bringing the total to 51.

A recent voice to join the chorus came Thursday, when Michigan Democrat Rashida Tlaib, who is on track to become the first Muslim woman in Congress, said she would "probably not" support Pelosi because "she doesn't speak about the issues that are important to the families of the 13th Congressional District, and they are a priority for me."

An additional 34 Democratic nominees are neither for nor against Pelosi, who has led her party in the House since 2003.

Later Friday, President Donald Trump, apparently prompted by the NBC News survey, tweeted, "Democrats please do not distance yourself from Nancy Pelosi."

Democrats, please do not distance yourselves from Nancy Pelosi. She is a wonderful person whose ideas & policies may be bad, but who should definitely be given a 4th chance. She is trying very hard & has every right to take down the Democrat Party if she has veered too far left! — Donald J. Trump (@realDonaldTrump) August 10, 2018

The significant opposition is a sign of the movement for a generational change in Democratic leadership on the Hill, where some believe that Pelosi should step aside so younger members of the party can move up in its ranks. The majority are Democrats running in Republican voting areas, where the minority leader is despised by the GOP. And some of it stems from the ascendant progressive movement, which wants to promote different policies and take a more aggressive approach in Congress to the Republicans and to Trump.

The anti-Pelosi contingent runs from competitive districts won by Hillary Clinton in 2016 to areas overwhelmingly carried by Trump. And among recruits in the Democratic Congressional Campaign Committee's "Red-to-Blue" program for targeted districts, 21 candidates say they will not vote for Pelosi as leader, 20 are neutral and four support her. NBC could not find positions for the other 11 candidates, and their campaigns did not respond to requests for comment.

For her part, Pelosi has not asked anyone to back her for House speaker should Democrats win the House in November, her office said.

"Leader Pelosi has always enjoyed the overwhelming support of House Democrats and that will continue into the majority she's so focused on winning," her deputy chief of staff, Drew Hammill, said Thursday when asked about the NBC News survey.

Though Pelosi sealed her place in history by becoming the first woman to serve as speaker, it's her longevity that also sets her apart.

Pelosi is the longest-serving leader in either party since Democrat Sam Rayburn and Republican Joseph Martin ruled Congress from the 1940s through the '50s. By the end of this year, three of her GOP counterparts will have come and gone: Dennis Hastert, John Boehner and Paul Ryan.

Pelosi was speaker from 2007 to 2011. If Democrats successfully take back the House and elect her as speaker, she would be the first to win the gavel back in over six decades. (Rayburn was the last, in 1955.)

Despite the opposition, there's still no clear alternative to Pelosi. Many candidates in the undecided category said they're waiting to see who, if anyone, comes forward to take her on — and only one could-be-contender has done so publicly so far.

Rep. Tim Ryan of Ohio, who challenged Pelosi for leadership after the 2016 election, indicated this week that he may consider another bid. "I've not closed the door on it," Ryan told MSNBC.

Pelosi has defended her stewardship of the Democratic caucus. "I'm a master legislator," she said last year, adding that the millions of dollars the GOP has spent "demonizing" her to voters shows that she's an effective leader.

And while NBC's research shows there are a sizable number of Democrats opposing Pelosi, the procedure for electing the leader can still provide an opportunity for stances to shift. After the 2016 election, 63 Democrats opposed Pelosi in a closed-door caucus meeting — but only four voted against her on the House floor.

The process tripped up Ohio Democratic candidate Danny O'Connor, who told MSNBC’s Chris Matthews that he would "vote for whoever the Democratic Party puts forward" on the floor. O'Connor later revised his statement to say he would oppose Pelosi in all circumstances, but that didn't stop Republicans from using it in an attack ad against him.

And while some Democratic House candidates, like O’Connor, have worked to cement their anti-Pelosi position, others, including New York's Alexandria Ocasio-Cortez, have softened in their stance.

Before her stunning primary victory over Rep. Joe Crowley in June, Ocasio-Cortez said she wanted to "see new options" in Democratic leadership. But afterward, she talked to Pelosi.

"She said she loved working with Joe, but that she's actually always wanted younger women in Congress,” Ocasio-Cortez told MSNBC's "Meet the Press Daily" of Pelosi. "I think she's a candidate to consider."