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WASHINGTON — If former national security adviser John Bolton is called to testify at the Senate impeachment trial, several Republicans told NBC News they believe that President Donald Trump will assert executive privilege.

A president claiming executive privilege during the trial would be unprecedented, and it's unclear how the Senate would handle the dispute. Chief Justice John Roberts is presiding over the proceedings and can rule on what evidence can be allowed — but his rulings can be overruled or sustained by a majority of the Senate.

Bolton's testifying would touch off concern in the White House because of his proximity to presidential decision-making, according to a senior administration official.

"It would be extraordinary to have the national security adviser testifying about his communications directly with the president about foreign policy and national security matters," the official said.

On Monday, a senior administration official told The Washington Post that one option being discussed would be to have Bolton testify in a classified setting.

Trump this month suggested he would claim executive privilege.

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"There are things that you can't do from the standpoint of executive privilege. You have to maintain that,” Trump said. "You can't have him explaining all of your statements about national security concerning Russia, China, and North Korea — everything — we just can't do that."

A source familiar with the trial planning confirmed the White House would likely want Bolton to testify behind closed doors. That would likely be the procedure regardless — the organizing resolution released by the Senate Majority Leader Mitch McConnell, R-Ky., on Monday calls for witnesses to be deposed behind closed doors before a decision is made on public testimony.

Some Republicans also said an attempt to get Bolton to testify could lead to a lengthy delay in the proceedings.

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Bolton "raises all sorts of issues. We're not a court of law, and if there is a claim of executive privilege or some national security privilege, one possibility would be that we'd need to adjourn the Senate trial for weeks if not months to allow that to be litigated through the district court, court of appeals, maybe even the Supreme Court," Sen. John Cornyn of Texas told NBC News.

Another Republican, Sen. Kevin Cramer of North Dakota, suggested the testimony wouldn't be worth halting the trial.

"I don’t want to see the trial stop for a week or two while we wait for a court to decide if somebody can testify," Cramer told NBC.

Bolton, who declined to testify or provide documents in the House impeachment inquiry, said this month that he is "prepared to testify" before the Senate if subpoenaed. Other witnesses have testified that Bolton was upset by the president's dealings with Ukraine.

Democrats have called for witnesses, including Bolton, to testify, and Sen. Mitt Romney, R-Utah, has said he'd "love to hear" what Bolton has to say.

Senate Minority Leader Chuck Schumer, D-N.Y., expressed confidence that Democrats would win if the question is kicked to the courts.

"If the White House tries to exert executive privilege on an order that passes by definition in a bipartisan way and is signed by the Chief Justice of the Supreme Court on a subpoena, I think that the courts would look very favorably on that," Schumer said Tuesday.