Former national security adviser John Bolton shook up the Trump impeachment standoff Monday with his announcement that, if subpoenaed, he is "prepared to testify" before a Senate trial. It's still not clear, of course, when or even if a trial might occur, since House Speaker Nancy Pelosi is withholding the articles of impeachment. But, if there is a trial, and if the Senate subpoenas Bolton, he won't fight.

Senate Minority Leader Chuck Schumer has demanded the trial include witnesses. Schumer specifically wants the Senate to hear testimony from Bolton, current White House chief of staff Mick Mulvaney, and two other White House officials, Robert Blair and Michael Duffey.

To testify or not to testify? The potential witnesses have a lot of thinking to do. Their situation is unprecedented. There is no guidebook to tell them what to do. The Supreme Court has not issued a decision covering their situation. In his statement on Monday, Bolton noted that, in the absence of judicial guidance, "I have had to resolve the serious competing issues as best I could, based on careful consideration and study." Here are some of the factors that he, Mulvaney, and other possible witnesses have had to weigh:

House vs. Senate

Neither Bolton nor Mulvaney testified before the House impeachment proceedings. Bolton did not receive a subpoena. Mulvaney defied one at the direction of the White House. Bolton's former deputy, Charles Kupperman, was subpoenaed and asked a court to decide whether he would be required to testify. At that point, House Democrats abruptly withdrew the subpoena. Pelosi, impeachment leader Rep. Adam Schiff, and others claimed that a court fight would take too long, given the urgency that Trump be removed from office as soon as possible. So, they dropped the issue.

A judge later declared the matter moot.

In the end, Democratic leaders, who had characterized Bolton, Mulvaney, and others as critical witnesses, did not lift as much as a finger to compel their testimony. Pelosi and Schumer held strategy sessions, and, after the House passed impeachment articles, Schumer demanded that witnesses whom the House did not even try to compel to testify absolutely must testify before the Senate.

The problem is, the questions surrounding potential testimony remain unanswered. If there was a problem requiring judicial resolution in the House, wouldn't there also be a problem when the Senate seeks testimony?

The answer is yes, but possibly in different ways. First, it seems extremely unlikely that a court would rule that the president's aides have a blanket immunity that would prevent them from even appearing before the Senate. But, at the same time, it seems likely that some, or perhaps all, of the conversations between Bolton and the president, or Mulvaney and the president, would be covered by executive privilege.

There are cases in the past, most notably a matter involving George W. Bush White House counsel Harriet Miers and chief of staff Josh Bolten, that covered similar issues. That case ended in a way that was inconclusive for Congress: The House held Miers and Bolten in contempt, and a judge ruled that they had to appear for testimony. But the ruling did nothing to prevent them from appearing and refusing to answer questions based on executive privilege. And then the case dragged on past the end of Congress's session, and the once-passionate issue petered out.

So, that is not exactly precise guidance for the current situation. In addition, the case was handled by the U.S. District Court for the District of Columbia and was not considered by either the circuit court of appeals or the Supreme Court, so it never made it to the appellate level. And, of course, it involved the House, not the Senate, and did not take place in the context of impeachment.

So, the question is: Does any of that apply to the Senate in an impeachment trial? The answer is not clear. But there seems little doubt that the White House, should it seek to bar the president's closest advisers from testifying, would have some legal options to pursue. It's not clear that they would be winning options, but they could involve some fighting in court as the Senate trial gets underway.

The Roberts Factor

House and Senate impeachment proceedings are entirely different from each other, and one of the biggest differences is the presence in the Senate trial of a representative of the judicial branch of government, in the person of the chief justice of the Supreme Court. The Constitution says the chief justice will "preside" over the Senate trial. On the other hand, the Constitution gives the Senate the "sole power" to try the impeachment. So, it is highly unlikely that Chief Justice John Roberts will take charge of the Trump impeachment trial. In the President Bill Clinton trial in 1999, then-Chief Justice William Rehnquist wrote that, "I did nothing in particular, and I did it very well." Roberts probably won't do much more.

But he will be there. And even if he is just a rubber stamp — another way of saying that is that he will give the imprimatur of the judicial branch to the Senate's actions. So, say the Senate, with the chief justice presiding, voted to issue a subpoena to Bolton, or Mulvaney, or anyone else. And say the recipient of the subpoena challenged it in court. How, exactly, would that work? Federal judges are not in the business of saying the chief justice of the Supreme Court is wrong. Would, say, Mulvaney take the subpoena to the U.S. District Court and ask a judge there to declare the Roberts-approved subpoena invalid? Perhaps. And perhaps he might succeed. But it's also possible the judge would be hesitant to overrule the chief justice. Roberts's role could make the decisions of lower court judges, in other words, the entire federal judiciary, more difficult.

Show up, yes. Answer questions, no

Finally, even if Bolton or Mulvaney or others were subpoenaed, and even if they challenged the subpoena in court, and even if they lost, there is a difference between showing up to testify and actually answering questions. There is no doubt that both men, and perhaps others, could honor a subpoena, appear, and then decline to answer a number of questions on the basis of executive privilege.

If executive privilege covers anything, it is a president's deliberations with his chief of staff and his national security adviser on the issue of foreign policy. The privilege is the president's, not Bolton's or Mulvaney's or anyone else's. Bolton's brief statement said that he is "prepared to testify," but it did not get into details of whether he would answer all questions or whether he would follow White House directions, if there were any, or what else he might or might not do.

As for Mulvaney, the Justice Department has already taken the position that he is "absolutely immune from compelled congressional testimony in his capacity as senior adviser to the president." That is a position a U.S. district judge has already rejected as applied to former White House counsel Don McGahn. But in the McGahn case, Judge Ketanji Brown Jackson made clear that her ruling was "solely" about whether a presidential aide is required to respond to a congressional subpoena by appearing in person. "The court distinguishes this issue from the very different question of whether the specific information that high-level presidential aides may be asked to provide in the context of such questioning can be withheld from the committee on the basis of a valid privilege," Jackson wrote.

That will be a critical question if Bolton, Mulvaney, or other witnesses appear before the Senate. Each will be allowed to have a lawyer present, and it seems likely that White House lawyers will be present, too. (The Democratic House did not allow administration lawyers to be present during testimony.) Those lawyers could raise privilege issues over and over and over, depending on what is asked. Testimony that is billed as dramatic, and even explosive, might be somewhat less exciting in reality.

Bolton's statement is an important development. But there is a lot more to the question of testimony in the Senate impeachment trial. It might not even happen — at least four Republican senators would have to join minority Democrats to make it possible. And even if it does, it could quickly turn into a very complicated affair.