Anticipating this strategy from the right, Democratic candidates are increasingly opposing her as speaker or refusing to take a stand an act of political inoculation — a trend that even her allies acknowledge could ultimately imperil her grip on the Democratic caucus.

At least two other senior Democrats, Representatives Steny H. Hoyer of Maryland and Joseph Crowley of New York, are already actively laying the groundwork to seek the leader’s job if Ms. Pelosi’s position becomes untenable.

As Democratic House candidates descended on Washington last week for a round of training and fund-raising, the topic of how to confront the Pelosi question came up repeatedly, according to multiple officials.

“I was just in D.C. and that’s the advice everybody gives: Don’t say you’re for Pelosi,” recalled former Representative Brad Ashford, a Nebraska Democrat trying to reclaim his seat. (He would not rule out backing Ms. Pelosi.)