Chief Justice John Roberts Jr. on Thursday refused to read aloud a question from Kentucky Republican Sen. Rand Paul that was widely reported as including the name of the anonymous whistleblower who first raised alarms over the Trump administration's interaction with Ukraine.

Paul reportedly wanted to ask a question that explored the origins of the U.S. House's investigation of President Donald Trump's withholding of funds from the Ukraine allegedly in return for an announcement that Ukraine was investigating former Vice President Joseph Biden and his son Hunter, who was serving on the board of a Ukraine corporation.

Roberts, according to a Politico report on Wednesday night, had "communicated to senators that he will not read aloud the alleged Ukraine whistleblower's name." Roberts said Thursday, after receiving a question from Paul: "The presiding officer declines to read the question as submitted."

Paul left the chambers on Thursday after Roberts refused to read the question, which reportedly did name the individual who has been identified in some media reports as the possible whistleblower. Paul on Thursday afternoon tweeted his question and the reasons he wanted to ask it.

House managers have refused to identify the whistleblower, arguing his identity is irrelevant to the proceedings. Senate Republicans also reportedly were not in favor of exposing the whistleblower during the impeachment trial. Sen. Charles Grassley, R-Iowa, long a supporter of whistleblower rights, has been the leading Senate proponent of legislation protecting whistleblowers.

Roberts has read aloud dozens of questions addressing a range of issues, including the standard for conviction, Roberts' own powers as presiding officer to issue subpoenas or make evidentiary rulings and the legality of House-issued subpoenas. On Wednesday, he read aloud part of the infamous "Access Hollywood" tape on which Trump said about touching women without their consent: "When you're a star, they let you do it—you can do anything."

Roberts' decision to reject the whistleblower-related question was his first substantive decision during the proceedings. Earlier in the week, he admonished lawyers for both sides to watch their language and remember "where they are."

"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.

But he may face a much more difficult decision on Friday, when the Senate is expected to debate—and vote on—whether to call witnesses, including John Bolton, the former national security adviser who has said he would speak about Ukraine matters, if subpoenaed.

A draft manuscript by Bolton, represented by Charles Cooper of Washington's Cooper & Kirk, reportedly says Trump directly told him he was withholding military assistance to Ukraine in order to get that nation's new president to announce an investigation of the Bidens.

Trump has raised the possibility of asserting executive privilege to block any testimony from Bolton. The National Security Council, in a prereview of the Bolton manuscript, reportedly told Cooper that the book contains a large amount of classified material. Cooper has denied that assessment.

Charles Cooper Charles Cooper, of Cooper & Kirk at the White House last year. (Photo: Diego M. Radzinschi / ALM)

If any Senate witness vote ends in a tie, Roberts may be asked to break it. Legal scholars are divided over whether Senate rules permit Roberts to cast such a vote. The chief justice could simply decide not to cast the tie-breaking vote, which means the motion to call witnesses would fail on a 50-50 vote.

Chief Justice William Rehnquist, who presided over the impeachment trial of President Bill Clinton, did not face any tie votes. The final Senate vote on the second article of impeachment against Clinton was 50-50, and that article failed.

In the only other impeachment trial—of President Andrew Johnson—then-Chief Justice Salmon Chase did vote to break two ties, but the Senate changed its impeachment rules after that event.