Nancy Pelosi, the speaker of the US House of Representatives, has announced a formal impeachment inquiry into President Donald Trump.
The Democratic-controlled House will examine whether the US leader sought Ukraine's help to smear former Vice President Joe Biden, the front-runner for the 2020 Democratic presidential nomination.
Ms Pelosi said the president's alleged actions represented “betrayal of his oath of office, a betrayal of our national security and betrayal of the integrity of our elections.”
Making the announcement on Tuesday night, marking the first step in a Democratic bid to remove the US leader, she said: "The president must be held accountable. No one is above the law."
Mr Trump, who is in New York meeting for a United Nations summit, has denied impropriety but acknowledged discussing political rival Mr Biden with the Ukrainian president.
Tweeting after Ms Pelosi's speech, he branded the inquiry a "witch hunt".
Ms Pelosi had for months resisted calls to officially start the impeachment process.
But the dynamic changed for the House Speaker this week as more members came out in support of the move, including Democrats from swing districts.
Democrats are demanding information about whether Mr Trump improperly pressured the Ukrainian president to investigate Mr Biden and his son, partly by withholding foreign aid.
The US leader, a Republican, insisted he did nothing wrong and said he will release an unredacted transcript of a call with Ukraine's leader on Wednesday.
President Trump had said that a formal impeachment inquiry by House Democrats would be a "positive for me."
He said the country is "doing the best it's ever done" and that Democrats are going to lose next year's elections if they pursue impeachment.
On Ms Pelosi's apparent move, he said: "If she does that, they say that's a positive for me."
Democrats are expected to explore whether ,Mr Trump pushed Ukraine's leader for help investigating Democrat Joe Biden and his son during a summer phone call.
Mr Trump said he had authorised the release of a transcript of the call on Wednesday.
