// ... where a "high crime" is usually understood to be a behaviour contrary to the duties of high office. //
I should perhaps add that merely being "contrary to the duties of High Office" is probably too vague, in fact, to be called a High Crime. It would, for example, be "contrary to the duties of high office" to steal the office stationery, but that would be manifestly unimpeachable even though it's the wrong thing to do. Still, behaviour "contrary to the duties of high office" isn't necessarily a legal crime, and refers to political misconduct. It's therefore nonsense that the White House has tried to head off a second Impeachment (and, for that matter, the first) by calling it "political". Of course it's political. That is why it falls to the Congress, not to any Court, to decide whether to Impeach, and whether to convict.
In 1999, Clinton was impeached after being accused of lying under oath and trying to obstruct justice. His was acquitted by the Senate, partly because the Democrats there en bloc voted against, and partly because even Republicans seemed somewhat split about whether to convict or not. Regardless, the point is that at the time Impeachment was described as being appropriate for "...[only] the most serious offenses, and in particular those that subvert [the US] system of government..."
I am not sure you can get more open-and-shut a case of "subverting the system of government" than whipping up a riot that marches on that Government in the hopes of forcing it to declare you the victor in an election, or in the hopes of intimidating it. Trump's behaviour surrounding the election has been disgusting. What is disheartening is that there are still those in that same Government that support him even today, even in spite of the fact that he effectively encouraged an attempt to overthrow them and put their very lives at risk.