In the predawn hours Wednesday, Chief Justice John Roberts, the presiding officer at the impeachment trial of President Donald Trump, asserted his voice forcefully for the first time during the Senate proceedings to warn the lawyers for both sides to watch their language.

The chief justice's remarks came after a terse exchange between Rep. Jerry Nadler, D-New York, one of the House managers, and Pat Cipollone, the White House counsel who is leading the Trump defense team.

Nadler at one point had accused the Trump White House of obstructing Congress. "They lie, and lie and lie and lie," he said. Cipollone described his counterpart as "the only one who should be embarrassed." He called on Nadler to apologize to Trump and to the country.

By then, after hours of wrangling over process and procedure, and with the clock well past midnight, Roberts had heard enough and spoke out.

"I think it is appropriate at this point for me to admonish both the House managers and the president's counsel in equal terms to remember that they are addressing the world's greatest deliberative body," Roberts said.

Roberts continued: "One reason it has earned that title is because its members avoid speaking in a manner and using language that is not conducive to civil discourse."

He pointed to an earlier impeachment trial that involved then-District Judge Charles Swayne of the Northern District of Florida. (Swayne was found not guilty in the Senate.)

"In the 1905 Swayne trial, a senator objected when one of the managers used the word 'pettifogging' and the presiding officer said the word ought not to have been used," Roberts said.

"I don't think we need to aspire to that high of a standard. But I do think those addressing the Senate should remember where they are," Robert said.

Schiff Rep. Adam Schiff, D-California, on Nov. 19, 2019. (Photo: Diego M. Radzinschi / ALM)

Earlier in Tuesday's proceedings, U.S. Rep. Adam Schiff of California, the lead House manager, argued that the president's attorneys are attacking the House managers in an attempt to distract from the substance of the claims against Trump. Schiff, speaking on the Senate floor, called the position of Trump's lawyers "ass-backwards"—the idea that there would be a trial without witnesses.

Roberts is expected to largely play a ceremonial role at the impeachment trial, ruling on procedural matters but not making any substantive decision about whether witnesses should be forced to testify. He is splitting his time between the Senate and the Supreme Court, where he is expected to preside this morning for a major case testing the limits of public student aid for religious schools.

Roberts is relying on one of his clerks, Megan Braun, and his counselor, Jeffrey Minear, for guidance during the proceedings.

Trump is on trial for charges of abuse of power and obstruction of Congress. He is accused of using the office of the White House to pressure a foreign power to announce investigations that would help him politically. His defenders largely have attacked the impeachment process itself, calling it "rigged."

The impeachment trial is expected to last at least until next week.