Nancy Pelosi is girding for a mortal challenge to her leadership of the Democratic Party after the November elections or even sooner, and in a signal of reassurance to anxious lawmakers says that she is deliberately building a “bridge” to a new generation of party leaders.

In perhaps the most serious test of her 15-year grip on the House Democratic caucus, Ms. Pelosi is facing unrest from older lawmakers critical of her style, younger Democrats demanding generational change and candidates across the party who have sought to inoculate themselves against Republican attacks by distancing themselves from her.

One of Ms. Pelosi’s deputies, Representative Jim Clyburn of South Carolina, said in an interview that he would seek the speakership if she struggles to amass the required 218 votes after November — an extraordinary acknowledgment of her vulnerability given the culture of deference in Washington.

Ms. Pelosi remains intent on reclaiming the speakership, but in an interview on Wednesday she also acknowledged a handover of power was coming eventually, and she encouraged would-be successors to prove their political mettle. Any aspiring party leader, she said, must demonstrate that “they do have a following, that they’ve shown a vision for the country,” as well as the necessary fund-raising prowess.