Majority Leader Mitch McConnell has said that he won't agree in advance to Democrats' demands for Trump administration witnesses in a Senate impeachment trial.

“The House may have been content to scrap their own norms to hurt President Trump, but that is not the Senate,” McConnell, a Kentucky Republican, said Monday. “Even with a process this constitutionally serious, even with tensions rising in the Middle East, House Democrats are treating impeachment like a political toy.”

McConnell and Minority Leader Chuck Schumer, a New York Democrat, can’t come to an agreement on the terms of a Senate trial, which has prompted Speaker Nancy Pelosi, a California Democrat, to hold onto the two impeachment articles House Democrats passed last month that accuse Trump of abuse of power and obstruction of Congress.

McConnell had planned to begin the trial in January but is now moving onto more judicial nominations during that time instead, while the Senate Finance Committee will begin consideration of a major trade deal among the United States, Canada, and Mexico.

Senators had anticipated the trial would begin in January and last several weeks. But the Senate has returned to legislation, and there is no clear indication when, or if, a trial will begin.

Schumer, speaking after McConnell on the Senate floor, justified the delay in the trial by arguing that holding the proceeding without an initial agreement on the witnesses Democrats demand would constitute a cover-up.

“At the moment, there is a very clear difference of opinion between the Republican leader and myself about what it means to have a fair trial,” Schumer said.

McConnell wants the Senate to hear the opening arguments from each side before holding a vote on witnesses, but Schumer and Pelosi want an agreement before the trial begins.

The Democrats said their case for witnesses has been bolstered by former national security adviser John Bolton’s announcement Monday that he would agree to testify before the Senate, if called.

“Previously, Mr. Bolton said he was leaving his question of testimony up to the courts, but today he made it perfectly clear that he’ll come if the Senate asks, as he should," said Schumer.

Democrats believe Bolton and other Trump officials can provide information showing the president acted to temporarily block security aid to Ukraine to coerce the country into pledging to investigate former Vice President Joe Biden and Democrats.

Schumer repeated his warning to the GOP that he would call up a vote on each of the four witnesses Democrats want to testify, including acting White House chief of staff Mick Mulvaney.

A simple majority would be needed to compel each witness to testify. Democrats control 47 votes. No Republican yet has pledged to vote to summon any of the witnesses on Schumer’s list.

“Make no mistake, there will be votes on whether to call each of the four witnesses we proposed,” Schumer said.