Ms. Pelosi’s allies have characterized the opposition as driven primarily by men, a feature underscored by the letter, which is signed by only two women, Representatives Kathleen Rice of New York and Linda T. Sánchez of California.
Among those who have called for fresh faces are members of the Congressional Black Caucus, who have said someone from their ranks should get one of the two top slots in the event of any vacancy. But no member of the group signed the letter, including Representative Marcia Fudge, 66, whose name was conspicuously absent.
Ms. Fudge said last week that she would consider running for speaker and had previously signed a draft of the defectors’ missive, according to Democratic aides who insisted on anonymity to discuss the process of gathering signatures. A spokeswoman for Ms. Fudge did not respond on Monday to an inquiry about why the congresswoman removed her name. But one person familiar with the discussions, who also spoke on condition of anonymity, noted that Mr. Ryan had removed his name from a similar letter in 2016, in preparation for his run against Ms. Pelosi.
Allies of Ms. Pelosi swung into action Monday, rallying to her side as her office trickled out endorsements from critical constituencies. Representative Jimmy Panetta of California put out a letter signed by eight other Democrats who are veterans in which they praised her commitment to their issues and her national security bona fides.
Representative Dwight Evans of Pennsylvania also released a letter, praising Ms. Pelosi’s “stamina” and crediting her with holding Democrats together on critical issues. Pelosi aides provided a copy of a letter from Harold A. Schaitberger, the general president of the International Association of Firefighters, who praised her “fortitude, leadership, tenacity and courage.”
Even as Ms. Pelosi braced for a challenge, Mr. Clyburn appeared to fend one off. Representative Diana DeGette of Colorado said she was dropping her bid to challenge him.
Her withdrawal means that in the face of escalating discussion about the need for fresh faces to lead Democrats, the tally of lawmakers who have declared their intentions to run against one of the top three leaders — all with a decade or more of service at the helm of their party — stands at zero.