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.
Elected to the House in 2018, Mr Van Drew pledged his "undying support" for Mr Trump.
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.
What did McConnell say?
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.
"This is by far the thinnest basis for any House passed presidential impeachment in American history. The thinnest, and the weakest. And nothing else even comes close," he said.
How did Senate Democrats respond?
Chuck Schumer, the Senate's leading Democrat, said Mr McConnell had conspicuously failed to defend President Trump's actions.
"The Republican leader could not rebut the accusations against the president. He did not advance an argument in defence of the president's conduct on the merits, that in itself is a damning indictment of the state of the president's defence," he said.
Mr Schumer said Democrats wanted a "fair and speedy" trial, with four witnesses being called and time limits on each stage of the process.
What happens now?
Democrats and Republicans must now agree how the Senate trial will be held.
Mr McConnell has already said there is no chance the Senate will convict Mr Trump. He has also indicated that he could use the Republican majority to end the trial early without witness testimony.
Mr Schumer however criticised the idea that the impeachment trial could take place without witnesses being heard.
"If the House case is so weak, why is leader McConnell so afraid of witnesses and documents?" he asked.
Meanwhile Democratic House Speaker Nancy Pelosi has suggested the House could delay sending the articles of impeachment to the Senate.
This could put off the trial for an indefinite period, denying Mr Trump his expected acquittal.
What is Trump accused of?
He is accused of pressuring Ukraine to dig up damaging information on one of his main Democratic challengers for the presidency in 2020, Joe Biden, and his son Hunter.
Hunter worked for a Ukrainian company when Joe Biden was US vice-president.
The president is accused of dangling two things as bargaining chips to Ukraine - withholding $400m of military aid to Ukraine that had already been allocated by Congress, and a White House meeting for Ukraine's president.
This, Democrats say, amounts to an abuse of presidential power, using the office for personal political gain and to the detriment of national security. Ukraine was using that money in its ongoing conflict with Russian-backed rebels.
Mr Trump is also accused of obstructing Congress by refusing to co-operate with the congressional inquiry.
Also on Thursday, Russian President Vladimir Putin dismissed the charges against Mr Trump as "absolutely made up".
"This is nothing but a continuation of an internal political struggle, with the party that lost the election, the Democratic Party, trying to reach its goal by different means," Mr Putin said during his annual end-of-year news conference.
Want to find out more?
- A SIMPLE GUIDE: If you want a basic take, this one's for you
- GO DEEPER: Here's a 100, 300 and 800-word summary of the story
- WHAT'S IMPEACHMENT? A political process to remove a president
- VIEW FROM TRUMP COUNTRY: Hear from residents of a West Virginia town
- CONTEXT: Why Ukraine matters to the US
- FACT-CHECK: Did Ukraine interfere in the 2016 election to help Clinton?
- CASE FOR & AGAINST: What legal scholars say about Trump conduct
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