They accused Democratic prosecutors of omitting key evidence when they presented their case.
The US President's lawyers are pressing the Republican-led chamber to acquit Trump of charges that he abused his power and obstructed Congress.
Trump's legal team planned an aggressive, wide-ranging defense asserting an expansive view of presidential powers and portraying him as besieged by political opponents determined to undo the results of the 2016 election and ensure he won't be reelected this November.
The lawyers also aim to put Democrat Joe Biden on the defensive as he campaigns for the lead off Iowa caucuses next month.
White House Counsel Pat Cipollone said: "They're asking you not only to overturn the results of the last election, but as I've said before, they're asking you to remove President Trump from an election that's occurring in approximately nine months."
He added: "They're asking you to tear up all the ballots across this country on their own initiative."
The opening of the defense's case comes after a three-day presentation by House Democrats.
As they wrapped up on Friday, they asserted that Trump will persist in abusing his power and endangering American democracy unless Congress intervenes to remove him before the 2020 election.
They also implored Republicans to allow new testimony to be heard before senators render a final verdict.
"Give America a fair trial," said California Rep. Adam Schiff, the lead Democratic impeachment manager.
Schiff closed Democrats’ case after methodical and impassioned arguments detailing charges that Trump abused power by asking Ukraine for politically motivated probes of political rivals, then obstructed Congress’ investigation into the matter.
Trump's lawyers contend Trump was within his rights as president when he asked Ukraine for the investigation.
House managers made the procession across the Capitol before the trial resumed Saturday to deliver the 28,578-page record of their impeachment case to the Senate.
The seven Democratic prosecutors peppered their arguments with video clips, email correspondence and lessons in American history.
Trump, with his eyes on the audience beyond the Senate chamber, bemoaned the trial schedule in a tweet, saying it "looks like my lawyers will be forced to start on Saturday, which is called Death Valley in T.V."
The president is being tried in the Senate after the House impeached him last month on charges he abused his office by asking Ukraine for the probes at the same time the administration withheld hundreds of millions of dollars in military aid.
The second article of impeachment against Trump accuses him of obstructing Congress by refusing to turn over documents or allow officials to testify in the House probe.