"So no one should express partisan certainty regarding President Trump’s suggestion that the Supreme Court might well decide that impeaching a president without evidence of high crimes and misdemeanors is unconstitutional."

Writes Alan Dershowitz, surprising me (and I taught the constitutional law relating to impeachment for many years). He writes:

Two former, well-respected justices of the Supreme Court first suggested that the judiciary may indeed have a role in reining in Congress were it to exceed its constitutional authority. Justice Byron White, a John F. Kennedy appointee, put it this way:

"Finally, as applied to the special case of the President, the majority argument merely points out that, were the Senate to convict the President without any kind of trial, a Constitutional crisis might well result. It hardly follows that the Court ought to refrain from upholding the Constitution in all impeachment cases. Nor does it follow that, in cases of presidential impeachment, the Justices ought to abandon their constitutional responsibility because the Senate has precipitated a crisis."

Justice David Souter, a George H. W. Bush-appointee, echoed his predecessor: “If the Senate were to act in a manner seriously threatening the integrity of its results … judicial interference might well be appropriate.”

It is not too much of a stretch from the kind of constitutional crises imagined by these learned justices to a crisis caused by a Congress that impeached a president without evidence of “high crimes and misdemeanors.” The president is not above the law, but neither is Congress, whose members take an oath to support, not subvert, the Constitution. And that Constitution does not authorize impeachment for anything short of high crimes and misdemeanors.

Here's the case he's talking about Nixon v. United States. — about a federal judge named Nixon who challenged the procedure the Senate used to convict him. All of the Justices rejected Nixon's attempted appeal to the judiciary. The Souter and White opinions were concurring opinions. The majority opinion written by Chief Justice Rehnquist (and joined by Stevens, O'Connor, Scalia, Kennedy, and Thomas) stressed the "textually demonstrable commitment" of the issue to the Senate, which is given the "sole Power to try all Impeachments." (The House is given "the sole Power of Impeachment.") Even though that case was about a judge, the Court took into account the special need for finality that would exist in the case of a President:

[O]pening the door of judicial review to the procedures used by the Senate in trying impeachments would "expose the political life of the country to months, or perhaps years, of chaos."... This lack of finality would manifest itself most dramatically if the President were impeached. The legitimacy of any successor, and hence his effectiveness, would be impaired severely, not merely while the judicial process was running its course, but during any retrial that a differently constituted Senate might conduct if its first judgment of conviction were invalidated. 

Now, I take Dershowitz seriously, but it's a question of how to take him seriously. He's not talking while there's any actual proceeding in the House or the Senate, certainly not one that's reached a conclusion creating an interest in finality and the avoidance of chaos.

Dershowitz is participating in what is only political dialogue, and he's criticizing expressions of partisan certainty about what the law is.

We're all just talking about impeachment, and that's what Dershowitz is doing too. If the question is Is there any way that the courts could get involved? and other lawyers are going on shows and writing op-eds to say, No, there's no way at all, the Supreme Court shut that down decisively in 1993 in Nixon v. United States, there's a place in this chattery dialogue for a lawyer to say, Oh, but there is a way.

By all means, get Dershowitz on your TV show if you can. Have him yell at the partisan certainty mongers and have them yell back at him. It's all theater. And you know as well as I do and Nancy Pelosi does that there isn't going to be a damned impeachment.