Senate allies of President Donald Trump are so concerned about the prospect of John Bolton testifying in Trump's impeachment trial that they're working with the president's legal team on contingency plans to prevent any such testimony from going public, The Washington Post reported Monday.

Under the plans, The Post said, if four Republicans choose to vote with Democrats and admit new witnesses to the trial, there would be moves to compel the witnesses to testify behind closed doors.

Bolton, the former national security adviser, is believed to have valuable insider knowledge about Trump's relations with Ukraine and has said he will testify if subpoenaed.

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President Donald Trump's Senate allies are so concerned about the prospect of John Bolton testifying at Trump's impeachment trial that they're working with the president's legal team on emergency plans, The Washington Post reported Monday.

Under the rules laid out by Senate Majority Leader Mitch McConnell, the House impeachment managers and Trump's defense team will present their arguments before senators in the opening days of the president's Senate impeachment trial.

Senators will then get to vote on whether new evidence should be admitted and new witnesses should be called.

Democrats would need four Republican votes to allow them to push for new witnesses, and The Post reports that McConnell thinks it could happen.

But if the Senate votes to allow new witnesses, The Post said, "McConnell is expected to ensure that those individuals are questioned in a closed-door session rather than a public setting."

Senate Majority Leader Mitch McConnell. AP Photo/J. Scott Applewhite

The Post said senators might move to take deposition from Bolton, the former US national security adviser, in "a classified setting because of national security concerns, ensuring that it is not public."

Senators party to the discussions also told The Post that if Bolton were issued a subpoena to testify, the White House would challenge his decision to appear in court, citing executive privilege.

Bolton is thought to have firsthand knowledge of Trump's covert campaign to try to get Ukraine to announce investigations into his political rivals.

No one knows exactly how damaging such knowledge might be, but with Bolton having said he will testify if subpoenaed, finding out would be a risk few Republicans seem willing to take.

The president's trial is scheduled to formally begin Tuesday in the Senate at 1 p.m. ET.