"Trials have witnesses" he said at a Capitol Hill news conference. "That's what trials are all about."
"Live testimony is the best way to go," he added, saying he was about to start negotiations with Senate Majority Leader Mitch McConnell.
Asked about Schumer's demands late Monday afternoon, McConnell responded: “We’ll be getting together, and we’ll have more to say on that tomorrow.” He didn't answer when asked when they would meet.
“Senate Democrats believe strongly the trial must be fair and it's very important that the American people judge it to be fair,” Schumer said. “A fair trial is one where senators get all the facts. And one that allows them to adjudicate the case impartially.”
Schumer, in a letter addressed to the McConnell on Sunday, asked to hear from four witnesses during the Senate’s impeachment trial – acting White House chief of staff Mick Mulvaney, former National Security Adviser John Bolton, Associate Director for National Security at the Office of Management and Budget Michael Duffey, and senior adviser to the acting White House chief of staff, Robert Blair – all of whom refused to participate in the House investigation.
“The four witnesses we propose have direct knowledge of why the aid to Ukraine was delayed,” Schumer said. “They might present exculpatory evidence that helps President Trump. It may be incriminating against the president. But they should be heard.”
Schumer also requested that the Senate issue subpoenas for a limited set of documents related to those decisions.
“I haven't seen a single good argument about why these witnesses shouldn't testify or these documents be produced, unless the president has something to hide, and his supporters want that information hidden,” Schumer said.
Schumer said that on Monday he sent the letter to every Republican and Democrat in the Senate, saying that he hopes that all of his colleagues will look at his proposed structure and “make up their own minds.” He said he expected Republicans would be "sympathetic" to his request because it would also help ensure a "speedy and fair" trial.
“A good number of my Republican colleagues have said they believe the charges are serious but there isn’t enough evidence yet for them to make a decision,” Schumer said.
Later in the press conference, Schumer said that a few Republicans have told him that they believe what Trump has done “is wrong” but they aren’t sure “enough facts have been presented to make an impeachable case, high crimes and misdemeanors.”
“We believe these witnesses and documents would provide the evidence they're looking for, without being dilatory and letting the trial drag on for too long,” Schumer said.
During the press conference, Schumer was asked about his previous position during former President Bill Clinton’s impeachment trial in the Senate in 1999. At the time, Schumer did not support calling witnesses in to testify. Their testimony was on videotape.
“The witnesses in '99 had already given grand jury testimony,” Schumer said Monday. “We knew what they were going to say. The four witnesses we called have not been heard from. That is the difference, and it is a difference that is totally overwhelming.”
“So to engage a trial without the facts coming out is to engage in a cover-up. To conduct a trial without the facts is saying we're afraid,” he said.
“There is no reason on God's green Earth why they shouldn't be called and testify unless you're afraid of what they might say,” he added.
Schumer also slammed his Republican counterpart for taking Senate impeachment trial negotiations public and for McConnell's insistence that Republicans are working in "total coordination" with the White House to defend Trump.
“It was very partisan, very slanted, very unfair,” Schumer said of McConnell’s televised interview last week.
McConnell, in an interview with Fox News' Sean Hannity on Thursday, said "everything I do during this, I'm coordinating with White House counsel," adding, "there'll be no difference between the president's position and our position as to how to handle this."
Schumer said that he tried launching negotiations with McConnell “two weeks ago” before McConnell went public with his statements. Schumer noted he sent the letter to McConnell over the weekend to “get things back on track” and to ensure a bipartisan and fair trial is conducted.
Schumer called the upcoming impeachment trial in the Senate an “enormously weighty and solemn responsibility.”
“It’s one of the most important things that the Founding Fathers gave to the Senate,” he said.
McConnell hasn't responded to Schumer's letter or his latest comments on Monday.
A spokesman to McConnell told ABC News in a statement about Schumer's letter: "Leader McConnell has made it clear he plans to meet with Leader Schumer to discuss the contours of a trial soon. That timeline has not changed.”
As for the timeline, Democrats proposed pretrial housekeeping measure be adopted on Jan. 6, followed by the swearing-in of the chief justice and senators on Jan. 7. House managers would make their presentations on Jan. 9, for no more than 24 hours, followed by the president's counsel, for no more than 24 hours.
Republicans, meanwhile, have mostly said they are not inclined to ask for witnesses to testify as part of the impeachment trial. Many GOP senators have said this would amount to a “circus” in the Senate and that they would rather have a shorter trial without any witnesses.
Trump and the White House have not been clear on whether they will insist on witnesses.
ABC News' Trish Turner contributed to this report.