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.
On Wednesday, Mr Trump said the votes in the House had been motivated by "envy and hatred and rage".
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.
He also 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.
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.
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.