US President Trump’s impeachment is the “most rushed, least thorough and most unfair” in history, the Senate’s Republican leader Mitch McConnell says.
The Democrat-led House of Representatives had let its “partisan rage” create a “toxic precedent that will echo into the future”, he added.
But Democrat Chuck Schumer said Mr McConnell had offered no defence of the president’s actions.
On Wednesday, the House voted to impeach Mr Trump on two charges.
The charges – that the president abused his power and obstructed Congress – centre on whether or not he improperly sought help from Ukraine to boost his chances of re-election in 2020.
Mr Trump now faces a trial in the Senate – but the Senate is controlled by the Republicans, so it is highly unlikely he will be removed from power.
Nearly all Democrats in the House of Representatives voted for the charges and every Republican against.
Congressman Jeff Van Drew was one of the few Democrats who broke ranks to vote against both articles of impeachment.
On Thursday, the New Jersey lawmaker formally announced his defection to the Republicans at a meeting with Mr Trump in the Oval Office, where the pair shook hands and posed for pictures.
Mr Trump, now only the third president in US history to face a trial in the Senate, decried the impeachment process as a “hoax” and a “set up”.
Looking ahead to the Senate trial, Mr Trump said White House lawyer Pat Cipollone, a 53-year-old Republican, is likely to represent him.
In his speech, Mr McConnell warned that the 12-week investigation could damage the institutions of US democracy and said the House had failed to prove that Mr Trump had broken any law.
Democrats had made up their mind to impeach Mr Trump before he was even inaugurated as president, he said, and theirs was “an impeachment in search of articles”.
The investigation of Mr Trump had taken far less time than the impeachment investigations into President Bill Clinton and President Richard Nixon, he said.