Calls are increasing for the Senate to subpoena former national security adviser John Bolton to testify in the impeachment trial, after a bombshell report indicated that President Donald Trump tied Ukrainian military aid to investigations on the Bidens during conversations with Bolton.

The New York Times on Sunday reported that a manuscript of Bolton's new book, circulated at the White House for review, states that Trump told his then-national security adviser the hold on Ukrainian military aid would not be released until the Biden investigations were announced. It's an explosive revelation that contradicts Trump's legal defense in the ongoing impeachment trial.

Bolton announced earlier this month that he would comply with a subpoena for his testimony in the impeachment trial. But Senate Republicans have largely stayed away from the prospect of subpoenaing Bolton over fears of a prolonged court fight, after Trump said he would likely claim executive privilege to fight an attempt to have the former White House aide testify.

There are still open questions about how Bolton could be called in as a witness, and parties have clashed in other court fights on whether a judge would even be able to rule on testimony from witnesses in impeachment proceedings. If a subpoena is issued, he could choose to comply and ignore any legal pushback from the Trump White House, as other witnesses in the initial impeachment inquiry have done.

Any fight over testimony would also put Chief Justice John Roberts in the spotlight. Senate Minority Leader Chuck Schumer said Monday that Roberts can make a ruling on witness testimony, and then be overruled by a Senate majority. He noted that Roberts, as Chief Justice William Rehnquist did during the Clinton trial, can defer to the Senate.

University of Missouri's Frank Bowman, a law professor and author of the book "High Crimes and Misdemeanors: A History of Impeachment for the Age of Trump," said he believes any privilege raised by Trump's lawyers would be addressed by the Senate, which is sitting as the court of impeachment.

"Which would mean, in this case, John Roberts gets the first crack at it," Bowman said. He said his guess is that Roberts would "avoid this like the plague," and would be "more disposed to rule that in the matter of an impeachment trial, questions of privilege are determined by the Senate itself."

As a potential court fight hangs over the Senate impeachment trial, here's what each party involved has already said about the issue.

White House counsel Pat Cipollone. (Photo: Diego M. Radzinschi/ALM)

The White House

Justice Department attorneys have argued in Trump's defense that, while even former top White House officials are "absolutely immune" from giving testimony to Congress, a court cannot rule on the issue.

In court filings made in the lawsuit filed by Charles Kupperman, Bolton's former deputy, over his subpoena issued in the House's impeachment inquiry, the DOJ lawyers wrote that a court cannot rule on the issue and that Kupperman should follow the White House's instruction not to comply with the subpoena.

"Never before in our nation's history has a senior White House official testified before Congress pursuant to judicial compulsion. That is no accident," a November court filing by DOJ states. "Leaving the president's senior-most aides open to compelled congressional testimony would allow Congress to encroach upon the independence and autonomy of the presidency by publicly interrogating the president's closest advisors to reveal the president's thinking or influence his decision-making on sensitive or controversial matters."

DOJ lawyers also argued the House can never go to court against the executive branch in other lawsuits, including one from the House Judiciary Committee seeking testimony from former White House counsel Don McGahn as part of the impeachment inquiry.

Justice Department attorney Hashim Mooppan told a panel on the U.S. Court of Appeals for the D.C. Circuit earlier this month that a court should not issue rulings related to an impeachment inquiry, or risk compromising the independence of the federal judiciary as a whole.

Douglas Letter, general counsel to the U.S. House of Representatives. (Photo: Jason Doiy/ALM)

House

House attorneys have been part of a legal battle for months over McGahn's testimony for impeachment purposes, arguing he is not "absolutely immune" from providing the subpoenaed information to Congress.

In the House Judiciary Committee lawsuit, House attorneys led by general counsel Douglas Letter, have questioned how else they would be able to obtain the information necessary for Congress' oversight powers, including impeachment, if the White House stonewalled them on the issue.

U.S. District Judge Ketanji Brown Jackson ruled last year that McGahn is not absolutely immune. She sharply criticized DOJ for its claims that the matter couldn't be resolved in court, saying those arguments "distort established separation-of-powers principles beyond all recognition."

House lawyer Megan Barbero also told the D.C. Circuit earlier this month that it should rule on the issue of impeachment testimony from McGahn.

"Even if there are political issues the courts will gladly avoid, it is the duty of the court in a case that is otherwise justiciable … to say what the law is," Barbero said at the time. "That is what we are asking the court to do."

Senate

In a court filing Friday, Senate lawyers told a judge at the U.S. District Court for the District of Columbia that they believe only the Senate should be permitted to set the rules of its impeachment trial.

Citing the U.S. Supreme Court case on the impeachment of Judge Walter Nixon, the Senate lawyers argued that "such procedural questions," such as whether to hear witness testimony "in an impeachment trial are textually committed to the Senate, and there are no judicially manageable standards to review a claim challenging these features of a Senate impeachment trial."

"The Constitution grants the Senate, and the Senate alone, the power to try impeachments and to determine its proceedings for doing so, and that power is not subject to review or interference by the Judiciary," the Senate filing states.

The brief was filed in a lawsuit from a Washington, D.C., attorney Martin McMahon, who sued last month over the procedures for the Senate trial. He argued that he and other Americans are being deprived of their right to a fair impeachment trial and sought the removal of GOP senators, including Senate Majority Leader Mitch McConnell, from the proceedings.