Sen. Lindsey Graham (R-SC) is worried that Republican attacks on former Trump national security adviser John Bolton may boost the chances that Bolton will be subpoenaed as a witness in the Senate impeachment trial.

Graham’s warning, in a statement issued Wednesday, comes as some of his GOP colleagues seek to “undermine Bolton’s credibility” after revelations that the manuscript of Bolton’s forthcoming tell-all book says “Trump directly tied the holdup of nearly $400 million in military assistance to Ukraine” to Ukrainian investigation of Joe Biden and his son, reports the Washington Post.

Observers on Twitter noted that Graham has previously said that Bolton’s claims don’t really matter to the impeachment trial, quoting Graham as saying that “one could assume everything attributable to John Bolton is accurate and still the House case would fall well below the standards to remove a president from office.”

Graham has, however, expressed support for obtaining Bolton’s manuscript from the White House to be reviewed by the Senate, behind closed doors.

Graham, one of Trump’s top allies in Congress, said last year that he won’t even pretend to be a “fair juror” in the impeachment trial: “This thing will come to the Senate, and it will die quickly, and I will do everything I can to make it die quickly.”

But the prospects for a quick acquittal now seem far less likely than they did last week.