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House Speaker Nancy Pelosi (D-Calif.) on Sunday evening notified her colleagues that the House "will proceed with bringing impeachment legislation to the floor" on Monday.

"In protecting our Constitution and our democracy, we will act with urgency, because this president represents an imminent threat to both," Pelosi wrote in her letter to House Democrats. "As the days go by, the horror of the ongoing assault on our democracy perpetrated by this president is intensified and so is the immediate need for action."

Pelosi said that before introducing an article of impeachment against President Trump, the House will seek a resolution calling on Vice President Mike Pence to invoke the 25th Amendment, giving him 24 hours to respond.

Rep. Ted Lieu (D-Calif.) tweeted that Pelosi's sequencing "is correct. Impeachment is one of the gravest powers of Congress. It should always be our last option. If [Trump] doesn't resign or if [Pence] doesn't invoke the 25th Amendment, then we will impeach. Tomorrow we introduce the article of impeachment."

Earlier Sunday, Lieu said more than 190 House Democrats have signed onto an article of impeachment accusing Trump of inciting an insurrection at the Capitol on Wednesday. Sen. Lisa Murkowski (R-Alaska) and Sen. Pat Toomey (R-Penn.) have both called on Trump to resign, with Toomey saying he thinks "the president has disqualified himself from ever, certainly, serving in office again. I don't think he is electable in any way."