Republican Donald Trump is likely this week to become the third US president to be impeached when the Democratic-led House of Representatives votes on charges stemming from his effort to pressure Ukraine to investigate political rival Joe Biden.
Trump faces one charge of abusing his power by asking Ukraine to investigate Biden, a leading Democratic contender to oppose him in the 2020 US presidential election, and one of obstructing Congress' investigation into the matter.
The president has denied wrongdoing.
"Impeachment is a hoax. It's a sham," Trump said on Friday. "There was nothing done wrong. To use the power of impeachment for this nonsense is an embarrassment to this country."
The House is likely to take up the issue on Wednesday, setting the stage for a vote this week on whether to approve the charges and send the matter to the Republican-led Senate to hold a trial on whether to remove him from office.
Democrats, who enjoy a 36-seat majority in the House, are expected to win an impeachment vote, which requires a simple majority.
Republicans hold 53 of the 100 seats in the Senate, where they appear likely to prevail in any trial against Trump, which would require a two-thirds majority to remove him from office.
Seeking to shape any trial, Senate Democratic leader Chuck Schumer called on Sunday for testimony from White House acting Chief of Staff Mick Mulvaney, former national security adviser John Bolton, Mulvaney aide Robert Blair and budget official Michael Duffey.
Schumer made his appeal in a letter to Republican Senate Majority Leader Mitch McConnell, who said last week he was coordinating with the White House and has raised the prospect of a short impeachment trial in which no witnesses would be called.
House Democrats also sought testimony from the four men in their inquiry, but they did not appear.
The White House did not immediately respond to a request for comment on the Schumer letter.
A McConnell spokesman did not directly address Schumer's requests, but said the Senate majority leader "plans to meet with Leader Schumer to discuss the contours of a trial soon."
'Misconduct hasn't stopped'
No US president has been removed as a direct result of impeachment.
Richard Nixon resigned in 1974 before he could be removed, while Andrew Johnson and Bill Clinton were impeached by the House, respectively in 1868 and 1998, but not convicted by the Senate.
The House Judiciary Committee voted 23-17 on Friday along party lines to approve the two charges against Trump and to send the matter to the full chamber.
Senior House Democrats expect to win any impeachment vote, albeit with the possibility of some defections from moderates facing tough re-elections next year in Trump-leaning districts.
In congressional hearings, Democrats have accused Trump of endangering the U.S. Constitution, jeopardizing national security and undermining the integrity of the Nov. 3, 2020, election by asking Ukrainian President Volodymyr Zelenskiy in a July phone call to investigate Biden and his son Hunter Biden, who was on the board of a Ukrainian gas company.
"It's a clear and present danger ... to our democracy," House Intelligence Committee Chairman Adam Schiff, whose panel spearheaded the investigation into Trump's actions, told ABC's "This Week" program on Sunday.
"The misconduct hasn't stopped," Schiff added, saying that Trump has still urged Ukraine, as well as China, to investigate the Bidens, and that Trump's personal attorney, Rudy Giuliani, recently traveled to Ukraine to conduct a "sham" investigation.
Republicans have defended Trump and accused Democrats of a politically motivated effort aimed at overturning his upset 2016 victory over Democrat Hillary Clinton.
Trump has alleged the Bidens were involved in corruption in Ukraine and should be investigated there, but has offered no evidence. Biden, a former vice president, has denied wrongdoing.