Speaker Nancy Pelosi of California threatened on Friday that the House could move to impeach President Trump over his role in inciting a violent mob attack on the Capitol if he did not resign “immediately,” appealing to Republicans to join the push to force him from office.

After a 3.5-hour call with fellow Democrats, Ms. Pelosi said she had instructed the Rules Committee to be prepared to move forward with either a motion for impeachment or legislation sponsored by Representative Jamie Raskin, Democrat of Maryland, to establish a body under the 25th Amendment that can declare a president is “unable to discharge the powers and duties of his office.”

“It is the hope of members that the president will immediately resign,” Ms. Pelosi said in a statement. “But if he does not, I have instructed the Rules Committee to be prepared.”

The announcement came after a call that Ms. Pelosi called “sad, moving and patriotic” in which members recounted the terror of the violent mob attack on the Capitol from Mr. Trump’s supporters.

“The violent insurrection was an attack on the caucus, the Congress, the country and the Constitution that was incited and facilitated by Donald Trump,” Representative Hakeem Jeffries, Democrat of New York and the caucus chairman, said on the call. “He must be held accountable for his actions.”

In a letter to House Democrats on Friday, Ms. Pelosi invoked the resignation of Richard M. Nixon amid the Watergate scandal, when Republicans prevailed upon the president to resign and avoid the ignominy of an impeachment, calling Mr. Trump’s actions a “horrific assault on our democracy.”

Ms. Pelosi also said she had spoken with Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, about “preventing an unstable president from initiating military hostilities or accessing the launch codes.”

A spokesman for General Milley, Col. Dave Butler, confirmed that the two had spoken and said the general had…