“All Classified National Security. Who would do this?” Trump concluded.

Bolton’s forthcoming White House memoir threw Trump’s impeachment trial into disarray after The New York Times reported over the weekend that an unpublished manuscript described the president telling his then-national security adviser that military aid to Ukraine was conditioned on the launch of probes into his domestic political rivals.

Those alleged revelations, which Bolton has not confirmed and Trump has denied, have forced Republican senators to confront intensified calls for testimony from Bolton, but the White House has warned them not to cave to Democratic demands for more witnesses.

The president’s invocation Wednesday of “Classified National Security” could bolster the argument from some GOP lawmakers that additional witnesses would tie the impeachment trial up in weeks-long court battles. The House, in crafting its articles of impeachment, opted not to call some witnesses because of legal fights over White House claims of executive privilege, another sticking point for Republicans who do not support further testimony.

Bolton’s attorney said he turned over a hard copy of the draft to the National Security Council late last month to ensure it did not inadvertently share classified information. An NSC spokesperson said the book remains under “pre-publication review” and that “no White House personnel outside NSC have reviewed the manuscript.”

Trump signaled earlier this month that he would invoke executive privilege if the Senate subpoenas Bolton, telling Fox News: “I think you have to for the sake of the office.”

The president again cited national security concerns with regard to witness testimony by administration officials when he appeared two weeks later at the World Economic Forum’s annual summit.

“When [Bolton] knows my thoughts on certain people and other governments, and we’re talking about massive trade deals and war and peace and all these different things that we talk about, that’s really a very important national security problem,” Trump told reporters in Davos, Switzerland.

Nearly 6 in 10 voters oppose the president’s invoking executive privilege to block new testimony in his trial, according to a POLITICO/Morning Consult poll released Wednesday.