Speaker Nancy Pelosi says the House of Representatives will file impeachment charges against US President Donald Trump for alleged abuse of power.
“Our democracy is what is at stake, the president leaves us no choice but to act,” the top elected Democrat said.
She spoke a day after the House Judiciary Committee began considering potential charges against the Republican president.
Mr Trump told Democrats to move quickly if they were going to impeach him.
He tweeted shortly before Mrs Pelosi’s remarks: “If you are going to impeach me, do it now, fast, so we can have a fair trial in the Senate, and so that our country can get back to business.”
The California congresswoman told Thursday morning’s news conference: “The facts are uncontested. The president abused his power for his own political benefit at the expense of our national security, by withholding military aid and a crucial Oval Office meeting in exchange for an announcement for an investigation into his political rival.”
She added: “Sadly, but with confidence and humility, with allegiance to our founders and a heart full of love for America, today I am asking our chairmen to proceed with articles of impeachment.”
Democrats are keen to hold a vote on impeachment in the House of Representatives before the end of the year, with the prospect of a trial in the Senate perhaps as early as January 2020.
White House press secretary Stephanie Grisham said immediately afterwards that the Democrats “should be ashamed”.
She added: “We look forward to a fair trial in the Senate.”
Earlier on Thursday, senior Trump adviser Kellyanne Conway told reporters: “We’re ready for a trial.”
“That’s when the defence goes on the offence,” she said, adding that Republicans look forward to calling their own witnesses.
On Wednesday, Mrs Pelosi held a behind-closed-doors meeting on impeachment with her fellow Democrats and asked them: “Are you ready?”
The lawmakers responded with a rousing “Yes”, according to the Associated Press news agency.