President Trump's impeachment trial is finally on the verge of being consequential.

The revelation that former National Security Adviser John Bolton's forthcoming book describes President Trump doing exactly what the administration and its defenders say he did not do (and which, they'll have you know, was perfectly good and right when he did it) has put the prospect of the Senate voting to permit witness testimony at the trial in a suddenly different light. A week ago it seemed wildly implausible that the GOP majority would permit Democratic impeachment managers to thus lengthen proceedings. Now, Sen. Pat Toomey (R-Pa.) is reportedly lobbying a few fence-sitting Senate Republicans to consider a witness deal.

Trump won't want it, and should his Senate bulwark fall, the next defense his team is expected to raise is the claim of executive privilege. This is where an impeachment trial may become something more than a foregone conclusion — it would gain the potential, in fact, to address an arguably bigger matter than the fate of Trump himself.

The concept of executive privilege does not appear in the Constitution. It is a creature both birthed and constrained by the Supreme Court, as University of Michigan law professor Barbara McQuade explained Monday at The Washington Post, intended to "promote frank and candid discussions between the president and his closest advisers without the chilling effect that could occur if they feared public disclosure of their conversations." Yet, she continues, it is "qualified and [required to] yield when outweighed by a compelling need, such as a criminal investigation." Trump isn't being tried for a crime in the ordinary sense, but only the surest partisan could claim there is no way impeachment might be a compelling need.

Naturally, the president's defenders have settled on precisely that strategy. While in the same breath claiming to exhibit "extraordinary and unprecedented transparency," the administration last year rebuffed a House impeachment inquiry request for testimony from Bolton and acting White House Chief of Staff Mick Mulvaney with an assertion of "absolute immun[ity]."

House Democrats didn't push back then. But now? Rep. Adam Schiff (D-Calif.), leading the House team of impeachment managers, has asked Chief Justice John Roberts, presiding over the Senate trial, to rule on the issue. What Trump's team fears, Schiff said Friday, "is that the justice will in fact apply executive privilege to [a] very narrow category ... and here that category may be nowhere at all, because you cannot use executive privilege to hide wrongdoing or criminality or impeachable misconduct."

Of course, as Schiff himself has acknowledged, the Senate could overrule Roberts' decision with a simple majority — a majority the GOP has. It isn't inconceivable that a few Republicans would cross the aisle to vote for bringing in witnesses only to defer to the White House on the executive privilege claim, likely taking cover behind a profession of concern for national security. Indeed, The Wall Street Journal editorial board is already waving the "national security" flag in its contention that a congressional definition of the boundaries of executive privilege would make the president but "a vassal of king Congress." That might be a persuasive caution except for, well, everything about our imperial presidency.

The Journal's recommendation is that House Democrats should have challenged Trump's privilege claims in court during the inquiry phase of the impeachment process. The possibility of a lengthy court battle now is just what many Senate Republicans are warning against. (A curious thought experiment: If a court case on privilege ran for months, would Senate Republicans abandon their loyalty to a Trump defeated in November? What happens to the "let the people decide" argument if the people decide for Trump's opponent?)

But it's not clear the privilege fight can still go to the courts: If the Senate voted to overrule Roberts and concede the administration's privilege assertion, that might be the end of it. "I think the Senate has the right to make its own rules and rulings," attorney James Robenalt, a Watergate expert, told The Hill. "That should keep the courts out of privilege decisions." He is not the only one who thinks so. For example, George Conway, lawyer and husband to White House counselor Kellyanne Conway, tweeted in November that "everything the Senate does [in this regard] you should assume to be judicially unreviewable."

Whether or not that assumption is correct, there's one more factor which could lead to a court battle after all: Has Trump waived his claim to executive privilege by publicly commenting on the very conversation he seeks to conceal? That's what McQuade argues at the Post. "Trump's tweets directly denying the substance of Bolton's reported allegations waive any privilege that might have protected them from public disclosure," she writes. "Privilege is meant to keep a president's secrets confidential. If the president reveals those secrets or publicly discusses the conversations himself, there is no longer any need to protect them from disclosure."

Trump's lawyers will undoubtedly disagree. If the Senate votes for witnesses and the administration duly offers privilege as its parry, we can expect a messy fight over how and on what grounds that dispute may be settled. Impeachment might actually get interesting.