The Senate’s top two leaders Tuesday engaged in a battle over the structure of President Trump’s impeachment trial, underscoring a new level of tension between the two sides before negotiations over the Senate proceedings even begin.

It started Tuesday as Senate Majority Leader Mitch McConnell (R-Ky.) rejected a call from his Democratic counterpart, Minority Leader Charles E. Schumer (N.Y.), to subpoena new witnesses for the proceedings, which are expected to begin next month, calling Schumer’s demands “dead wrong” and suggesting he didn’t understand the Constitution.

Asked during a later interview with The Washington Post about the contentious and public tit-for-tat between the two leaders before a formal meeting has even been scheduled, Schumer said McConnell was the one creating the tensions.

“I don’t do it,” he said. “He’s the one who does it.”

He argued that Republicans have not given “one solid reason, one simple reason” why a slate of Trump administration witnesses demanded by Democratic senators should not appear during next month’s all-but-certain impeachment trial.

“You know, when you don’t have the facts, when you’re afraid to argue why these witnesses shouldn’t be here, you look for one diversion or another,” Schumer said.

The dynamics playing out now are far different from the tone set by the two Senate leaders during the impeachment trial of President Bill Clinton. At that time, then-Sens. Trent Lott (R-Miss.) and Thomas A. Daschle (D-S.D.) made a concerted effort to protect the integrity of the Senate and negotiated a bipartisan proposal that had the unanimous backing of the chamber on some parameters of Clinton’s trial.

Two decades later, the tensions between McConnell and Schumer are quickly spilling into the open. Speaking on the Senate floor, McConnell said the request by Schumer, made Sunday, was “dead wrong” and suggested that the House’s case against Trump is “deficient.”

“We certainly do not need jurors to start brainstorming witness lists for the prosecution,” McConnell said, referring to the role of senators during the trial.

The remarks came in response to a letter from Schumer in which the Democrat outlined several procedural demands that he said would make an impeachment trial more fair. The demands included subpoenas for acting White House chief of staff Mick Mulvaney, Mulvaney senior adviser Robert Blair, former national security adviser John Bolton and top Office of Management and Budget official Michael Duffey.

Mulvaney, Blair and Duffey defied subpoenas from House committees, while Bolton, who was not subpoenaed, has said he would fight one in court.

“The fact that my colleague is already desperate to sign up the Senate for new fact-finding … suggests something to me,” McConnell said of Schumer. “It suggests that even Democrats who do not like this president are beginning to realize how dramatically insufficient the House’s rushed process has been.”

McConnell then argued that the Senate shouldn’t be in the business of building a case for impeachment, a job he said is the responsibility of the House.

“If House Democrats’ case is this deficient, this thin, the answer is not for the judge and jury to cure it over here in the Senate,” he said. “The answer is that the House should not impeach on this basis in the first place.”

Schumer dismissed that argument in The Post interview, insisting he believes the House has done a “great job.” He also did not rule out calling for additional witnesses such as Rudolph W. Giuliani, the president’s personal attorney who recently traveled to Ukraine and has recently defended the push to oust Marie Yovanovitch, the former U.S. ambassador in Kyiv.

“We haven’t limited our list to these four,” Schumer said, referring to Mulvaney, Bolton, Blair and Duffey. “But these four should and must come.”

Schumer also expressed confidence that the four administration officials would comply with a subpoena issued by the Senate, even after they bucked calls from the House to appear.

“I think a subpoena from the Senate that is bipartisan would be very hard to defy,” he said.

The House is on track to vote to impeach Trump on Wednesday, sending articles of impeachment for abuse of power and obstruction of Congress to the Senate. Under the Constitution, the Senate would hold a trial with all 100 senators as jurors. A two-thirds vote would be required for a conviction.

The Senate trial is expected to begin in January. McConnell and Schumer could begin hashing out its scope as early as this week, although a meeting has yet to be scheduled.

A McConnell-Schumer accord over witnesses appears unlikely. Senior Republicans hope to limit witnesses in a trial and have questioned why Schumer wants to negotiate all the structural elements, including witnesses, up front.

That wasn’t the case in 1999, when the Senate unanimously agreed on the length of opening arguments allotted for the House impeachment managers and Clinton’s defense team, as well as time for questions — but did not decide on witnesses until weeks into the trial.

“We know what the opening arguments are going to be. Let’s have the debate on witnesses now,” Schumer said. “I think they’re just trying to push it off and hope people forget about it, but we’re not going to forget about it.”

In a rebuttal on the Senate floor shortly after McConnell spoke, Schumer signaled that Democrats would force votes during the trial on calling witnesses and making public documents they have requested from the administration.

During the interview, Schumer also stressed that he expected all of his Democratic senators to participate fully in the Senate trial, even though five of them are campaigning for the presidential nomination just weeks before the first nominating contests begin in Iowa.

“I’ve made it clear that this is a weighty, somber … very important proceeding, and scheduling is not going to interfere,” Schumer said. “Not a single presidential has said, ‘Please don’t schedule it here, there. Please don’t do it quick.’ They’ve been very responsive.”