At the time, Bolton appeared to be saying that he would testify only if a judge ordered him to do so. But earlier this month the former Trump adviser shifted his stance and said he would appear and testify if subpoenaed in the Senate impeachment trial of his former boss.

A report in The New York Times on Sunday evening could make Bolton an even more compelling witness. The article said that the unpublished manuscript of a book by Bolton revealed that Trump told his former national security adviser in August that he wanted to continue withholding aid to Ukraine until the country carried out investigations into the president’s rivals.

Bolton‘s willingness to be a witness could be a crucial distinction from the others. If Trump wants to stop Bolton’s testimony, he may have to take legal action to halt it, while in the case of the other officials, the status quo suggests they will fight or ignore any Senate subpoena.

“Executive privilege cannot be used to prevent a witness who is willing to testify from appearing, and certainly not one who no longer works in government,” Rep. Zoe Lofgren (D-Calif.), one of the House impeachment managers, wrote on Twitter on Thursday. “It’s not a gag order. And witnesses testify on national security all the time. Bolton has a right to testify if he wants to.”

Some Bolton associates say they doubt he’d defy specific claims of executive privilege by the president.

“I know John Bolton and he’s a strong constitutionalist, so I don’t think he’d do that,” said David Rivkin, a former Justice Department official, while acknowledging that Bolton’s shift earlier this month was hard to interpret.

Democrats have also demanded documents that the White House could assert executive privilege over if the Senate subpoenaed them. Democratic senators could add to the list of potential witnesses they began demanding last month, but might refrain from doing so to avoid fueling claims that allowing any witnesses would bog the Senate down in an unwieldy mess.

Who would decide whether Trump’s privilege claims are valid?

If Trump’s lawyers object in the trial to any subpoena the Senate might issue, Chief Justice John Roberts could be the one to rule at the outset. Such a decision would be closely scrutinized, in part because Roberts and Trump have tangled publicly before about Trump’s blunt criticism of federal judges.

Whatever ruling Roberts issues would be subject to immediate appeal to the full Senate, where the vote might well be a close one, tracking any earlier vote to hear from witnesses in the first place.

Democrats seem hopeful that Roberts’ decisions would carry some weight.

“Whether or not there’s executive privilege related to a witness, he can rule right away,” Sen. Debbie Stabenow (D-Mich.) said on Fox News on Saturday.

Rep. Adam Schiff (D-Calif.), the lead impeachment manager and chairman of the House Intelligence Committee, told reporters on Friday: “Unlike in the House, where the president could play rope-a-dope in the courts for years, that is not an option for the president’s team here. We have a very capable justice sitting in that Senate chamber, empowered by the Senate rules to decide issues of evidence and privilege.”

Any of the witnesses or perhaps Trump himself could also try to take the dispute to court, filing a motion to quash the subpoenas in U.S. District Court in Washington.

A judge would probably take submissions from both sides before hearing arguments and issuing a ruling. That decision could be appealed to the D.C. Circuit Court of Appeals and even leap-frogged to the Supreme Court, if the justices agreed.

Taking the privilege issue to the courts before objecting in the Senate might be a good option if Trump’s lawyers want Roberts to rule on any disputes that reach the court. He might be reluctant to rule on challenging a decision he made as the presiding officer in the Senate.