“According to the testimony given to Congress so far, Bolton was a central figure in trying to prevent any delay in releasing foreign aid to Ukraine,” said John Yoo, a University of Berkeley law school professor and senior Justice Department official under President George W. Bush. “I cannot see how any responsible investigation would not seek Bolton’s appearance.”

But he added that the White House would presumably “go to the mat” to fight any effort to interview Mr. Bolton. “If the White House were to fight the House impeachment on executive privilege grounds, Bolton would be the hill on which to die,” Mr. Yoo said. “The Trump White House could claim not just that the impeachment investigation is illegitimate, which is its current line of defense, but that it is defending the right of future presidents to have an effective White House and to conduct a successful foreign policy.”…

The president’s defenders dismiss the idea that Mr. Bolton could hurt Mr. Trump. “I don’t care what Bolton says,” Senator Lindsey Graham, Republican of South Carolina and a close ally of the president’s, said on Fox News on Thursday. If the Ukrainians did not know the president had held up their aid when he was pressing them to investigate Democrats, Mr. Graham said, there is no impeachable offense. “You can’t have a crime unless you have a victim. There is no victim here.”