Democrats have announced two articles of impeachment against Donald Trump — abuse of power and obstruction of Congress — pushing towards historic votes over charges he corrupted the US election process and endangered national security.
House of Representatives speaker Nancy Pelosi, flanked by the chairmen of the impeachment inquiry committees, stood at the Capitol for what she called a "solemn act".
Voting is expected in days on the Judiciary Committee and by Christmas in the full House. Mr Trump insisted he did nothing wrong and his re-election campaign called it "rank partisanship".
"He endangers our democracy, he endangers our national security," said Jerrold Nadler, the Democratic chairman of the Judiciary Committee, as he announced the charges. "Our next election is at risk. That is why we must act now."
The president tweeted ahead of the announcement that impeaching a president with a record like his would be "sheer Political Madness!"
The House has only voted three times in history against a president. Approval of the charges would send them to the Senate in January, where the Republican majority would be unlikely to convict Mr Trump.
Democratic leaders say he put his political interests above those of the nation when he asked Ukraine to investigate his rivals, including Democrat Joe Biden, and then withheld 400 million dollars in military aid as the US ally faced an aggressive Russia.
They say he then tried to obstruct Congress by stonewalling the House investigation.
In drafting the articles of impeachment, Ms Pelosi faced a legal and political challenge of balancing the views of her majority while hitting the constitution's bar of "treason, bribery or other high crimes and misdemeanours".
Some liberal legislators wanted more expansive charges encompassing the findings from former special counsel Robert Mueller's probe of Russian interference in the 2016 election.
Centrist Democrats preferred to keep the impeachment articles more focused on Mr Trump's actions towards Ukraine.
Asked during a Monday evening event if she had enough votes to impeach the Republican president, Ms Pelosi said she would let House legislators vote with their conscience.
"On an issue like this, we don't count the votes. People will just make their voices known on it," she said. "I haven't counted votes, nor will I."
Mr Trump, who has declined to mount a defence in the House hearings, tweeted on Tuesday just as the six Democratic House committee chairmen prepared to make their announcement.
"To Impeach a President who has proven through results, including producing perhaps the strongest economy in our country's history, to have one of the most successful presidencies ever, and most importantly, who has done NOTHING wrong, is sheer Political Madness! #2020Election," he wrote.
The president also spent part of Monday tweeting against the impeachment proceedings. He and his allies have called the process "absurd".