The questions made public that may be asked of President Donald Trump, should he testify in the Robert Mueller investigation, suggest the special counsel is honing in on criminal activity in one or two realms, or both.
The first is whether Mr. Trump had the intent to obstruct justice by allegedly asking then-FBI Director James Comey to let then National Security Adviser Michael Flynn off the hook for lying about meeting with the Russian ambassador.
The second is whether Mr. Trump had committed criminal acts in business dealings uncovered in the seizure of papers and other evidence in the Justice Department raid on the home, office and hotel room of longtime Trump private lawyer Michael Cohen.
Both realms raise the possibility of what some lawyers have called a "constitutional crisis" involving the separation of powers. It is unclear, they say, whether the president can ever be obliged to testify by subpoena before a legislative body, and indeed whether he can be convicted of a crime.
In 1999, President Bill Clinton did testify and was impeached by a majority vote of the Republican-controlled House of Representatives in the notorious Monica Lewinsky sex scandal.
Two former presidents, Democrat Jimmy Carter and Republican Gerald Ford, had proposed that Mr. Clinton be censured on the condition he would admit he had lied to a grand jury. Instead, the Senate trial went forward and Mr. Clinton was acquitted by enough fellow Democrats sitting as jurors to keep the vote short of the two-thirds majority required for conviction.
Mr. Clinton, thus saved by party colleagues who deplored his conduct, shamelessly cast his acquittal as a defense of "the Constitution and the presidency," and finished his second term on an upswing.
Some 23 years later, coming into play again is the textbook concept that no individual in the American political system is, as the popular expression puts it, "above the law."
Mr. Trump's current team of lawyers, now led by former New York Mayor Rudy Giuliani, might want to call as a witness, if she's still around, the young secretary in the first George Bush administration, Fawn Hall. In 1988, in the Iran-Contra scandal, she famously declared that "sometimes you have to go above the law," after having smuggled some classified papers out of the White House.
The big question now is whether President Trump, who earlier had expressed willingness, even eagerness, to meet with Mr. Mueller, will now be ready to do so. His latest tweets have suggested otherwise, although two previous lead lawyers are said to have resigned because their client had seemed to be ignoring their advice against meeting with Mr. Mueller.
The list of more than 40 questions that Mr. Mueller is supposedly waiting to ask Trump may not be the best guide of what is to come. More than a month has passed since such an inquiry was conceived, and who knows what else Mr. Mueller and his investigators may have come up with since then.
Also, given Mr. Trump's penchant for rambling off the cuff, any such open-ended interrogation could turn out to be risky territory for him to enter in the hands of skilled veteran inquisitors.
Mr. Giuliani alluded to the possibility by saying on Fox News that what the investigators were "really trying to do is trap him into perjury, and we're not suckers." He insisted that two or three hours of Trump testimony should be sufficient, if any at all.
Impeachment itself may be too high a bar for Mr. Trump's political foes to clear right now, to say nothing of the even higher bar of the two-thirds majority in the Senate required to convict.
The only two presidents ever impeached, Andrew Johnson and Mr. Clinton, were acquitted. Richard Nixon, who likely would have been the first convicted, resigned instead to escape that humiliation. Would Donald Trump do likewise if faced with that same alternative?
It's hard to imagine this most boastful and self-regarding president taking the high road, no matter what the ultimate circumstances. But in the end he may have no choice but to resign, if only to salvage his opulent lifestyle out of office.
Jules Witcover is a syndicated columnist and former long-time writer for The Baltimore Sun. His latest book is "The American Vice Presidency: From Irrelevance to Power" (Smithsonian Books). His email is email@example.com.