Washington CNN  — 

John Roberts had been in the chair, presiding over the Senate impeachment trial for nearly 12 hours when he had had enough. He suddenly asserted his presence and made clear his rigorous sense of decorum.

Roberts told President Donald Trump’s lawyers and the Democratic lawmakers representing the US House to stop being so petty. But he did it in lofty terms and with a characteristic flourish of history.

The brief but potent moment reflected the by-the-book, mannerly character of the current chief justice of the United States. It also underscored the limited role Roberts has in the Senate, as opposed to his dominant place at the Supreme Court, where he has held the center chair since 2005.

While he will not steer the ultimate resolution of Trump’s fate, he will try to keep everyone in line as they get there.

Under US Constitution and Senate rules, Roberts is not deciding substantive issues or the fate of Trump; rather he is a “presiding officer,” enforcing procedures and maintaining the dignity of the session.

“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 at nearly 1 a.m. ET Wednesday, apparently intent on scolding both sides equally.

“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.”

And then in classic Roberts fashion, he had a case citation for that. He referred to the century-old Senate impeachment trial of Judge Charles Swayne of Florida.

“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.”

His admonishment came on the heels of a sharp exchange between House Judiciary Chairman Jerry Nadler and White House counsel Pat Cipollone over a subpoena for former national security adviser John Bolton’s testimony, who was involved in Trump’s dealings with Ukraine security aid.

Referring to the rejection of amendments for documents and witnesses, Nadler accused Republican senators of “voting for a coverup.”

“So far, I’m sad to say, I see a lot of senators voting for a coverup. Voting to deny witnesses, an absolutely indefensible vote, obviously a treacherous vote,” Nadler said. “A vote against an honest consideration of the evidence against the President. A vote against an honest trial. A vote against the United States.”

To that, Cipollone responded said Nadler made “false allegations” against the President and lawmakers.

“The only one who should be embarrassed, Mr. Nadler, is you. For the way you addressed this body,” Cipollone said. “This is the United States Senate. You’re not in charge here.

Until then, Roberts, who turns 65 next week, had mainly been keeping the clock, announcing vote tallies and engaging in all the procedural functions that define his ministerial role.

But he clearly was ready for the possible nastiness, having pored over all past Senate impeachment trials. Most have been of federal judges. Trump is only the third president subject to impeachment and trial.

And he has in recent years tried to separate himself and the federal judiciary from the politics of the day.

In November 2018, he implicitly admonished Trump for deriding a US district court judge who had ruled against his administration in an immigration case as an “Obama judge.” Roberts responded, “We do not have Obama judges or Trump judges, Bush judges or Clinton judges. What we have is an extraordinary group of dedicated judges doing their level best to do equal right to those appearing before them.”

Democratic House members have been hoping Roberts would take a more active role, and in fact one of the amendments that the Senate majority blocked would have empowered him to control the introduction of witnesses and evidence.

Roberts’ introduction to the Senate may have mirrored that of his predecessor as chief justice, William Rehnquist, who oversaw the Bill Clinton impeachment trial in 1999.

At the end of the trial, Rehnquist described his experience.

“I underwent the sort of culture shock,” he told senators, “that naturally occurs when one moves from the very structured environment of the Supreme Court to what I shall call, for want of a better phrase, the more freeform environment of the Senate.”