The speaker has been on a campaign to make the case against impeachment, saying in private and public that the move is the most divisive that Democrats could pursue. Instead, she has hammered away at a three-pronged strategy for challenging Mr. Trump — “legislate, investigate, litigate”— by pressing forward with measures to check his power and secure elections, scrutinizing his conduct and his administration’s policies, and taking legal action to compel his inner circle to answer to Congress.

But as she has brushed off suggestions she is taking heat within her own party, Ms. Pelosi has sharpened her own messaging against Mr. Trump, almost in direct proportion to the fervor within her ranks for impeaching him, telling reporters that he does not know right from wrong and “is involved in a criminal conspiracy.”

People close to the speaker likened her effort to the one she undertook during the midterm elections, when she resisted calls to make the races about Mr. Trump and instead encouraged Democrats to keep a single-minded focus on health care and other issues that polls showed were overwhelmingly popular with voters.

Representative Katie Hill, another freshman from a competitive district in California, said she was comfortable holding off on impeachment for now, but she has been spending more time educating people about the process.

“We’re getting more and more calls for impeachment, but when I explain it to people about these are the steps that we’re taking for accountability, then people understand,” Ms. Hill said last week in an interview. “They mainly want to know that we’re making forward movement. Many people are in a similar boat as I am, where if he takes it another step — if he defies a court order, or if and when we find evidence of underlying crimes — then we’re going to have to reassess.”

For her part, Ms. Porter acknowledged that she campaigned for office on lowering prescription drug costs, addressing the lack of affordable housing in Orange County and other policy issues — not impeaching Mr. Trump, despite his relatively weak support there. She framed her decision, though, as consistent with the values she presented to voters.

“When faced with a crisis of this magnitude, I cannot with a clean conscience ignore my duty to defend the Constitution,” Ms. Porter said in the video. “I can’t claim to be committed to rooting out corruption and putting people over politics and then not apply those same principles and standards in all of the work I do.”