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.
On Wednesday, the House voted to impeach Mr Trump on two charges – that the president had abused his power and that he had obstructed Congress.
He now faces a trial in the Senate.
However, the Senate is controlled by the Republicans, so it is highly unlikely Mr Trump 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 to Congress, Mr McConnell warned that the 12-week investigation could damage the institutions of US democracy and said that the House had failed to prove that Mr Trump had broken any law.
He has already indicated that he is ready to co-operate with the White House to ensure a swift trial for the president in the Senate.
But Democrats are unhappy at the way the trial could be held. House Speaker Nancy Pelosi has now indicated it might delay sending the articles of impeachment to the Senate, in order to bargain on the terms of the proceedings.
This could put off the trial for an indefinite period, denying Mr Trump his expected acquittal.