Former national security advisor John Bolton criticized the House Democrats’ impeachment effort against President Donald Trump as “grossly partisan” and said his testimony would have still resulted in the president’s acquittal.

Speaking Wednesday at Vanderbilt University alongside Susan Rice, former national security advisor under President Barack Obama, Bolton condemned House Democrats for what he described as “impeachment malpractice” and speculated that the proceedings “drove Republicans who might have voted for impeachment away because it was so partisan.”

Bolton shot to the top of the Democrats’ witness wish list after a New York Times report alleged that his forthcoming book — The Room Where It Happened: A White House Memoir — contains allegations that President Trump wanted to tie U.S. security assistance to Ukraine to investigations into former Vice President Joe Biden, and his son, Hunter Biden. President Trump has repeatedly denied the report and labeled the book “nasty & untrue.”

“People can argue about what I should have said and what I should have done,” Bolton said. “I would bet you a dollar right here and now, my testimony would have made no difference to the ultimate outcome.”

For anyone saying he should just “spill his guts” on what he knows, Bolton cited the “implied threat of criminal prosecution” if what he shares is determined to be classified information. Asked if he would have testified under a House subpoena, Bolton again cited the review process.

“I’m not here to speculate on that with the pre-publication review process under way,” Bolton said, drawing some laughs from the audience. “Laugh all you want. This is the judgment of my counsel, somebody I worked with 35 years ago, 30 years ago at the Department of Justice.”

House Speaker Nancy Pelosi (D-CA) and House Intelligence Committee Chairman Adam Schiff (D-CA) have put off — but not ruled out — a subpoena for Bolton, who refused to participate in the House impeachment inquiry but later said he would testify in the Senate trial.

The Associated Press contributed to this report.