One option being discussed, according to a senior administration official, would be to move Bolton’s testimony to a classified setting because of national security concerns, ensuring that it is not public.

To receive the testimony in a classified session, Trump’s attorneys would have to request such a step, according to one official, adding that it would probably need the approval of 51 senators.

But that proposal, discussed among some Senate Republicans in recent days, is seen as a final tool against Bolton becoming an explosive figure in the trial. First, Republicans involved in the discussions said, would come a fierce battle in the courts.

Trump’s trial begins in earnest Tuesday on the two impeachment charges: abuse of power and obstruction of Congress. They center on the allegation that Trump withheld military aid and a White House meeting to pressure Ukraine to investigate his political rivals, including former vice president and 2020 candidate Joe Biden. The Trump administration stonewalled the House impeachment probe, denying witnesses and documents.