An early warning of the challenge that would face the impeachers was contained in the second volume of Mueller’s report. There, Mueller notes that the obstruction-of-justice statute requires corrupt intent as an element of the crime, along with obstructive acts in a relevant investigation or proceeding. He notes that corrupt intent can often be inferred from the alleged conduct itself, but where there is a plausible legitimate governmental motive for an official’s conduct, that inference might not be possible. In such cases, the element of corrupt intent must be established separately—a difficult evidentiary challenge. Mueller left the issue of Trump's intent up to Congress.

At a certain level, it is eminently sensible to leave the question of what constitutes “Treason, Bribery, or other High Crimes and Misdemeanors” up to the House and the Senate. At the Constitutional Convention, James Madison objected to including “maladministration”—poor governance—among the causes of impeachment because, he said, that would amount to the president serving at the pleasure of the Senate. But that is precisely where we have ended up, and that’s not a bad thing: The requirement of a two-thirds majority guarantees that removal by the Senate will be not a partisan exercise, but a matter of broad public judgment.

In the current Congress, for example, removing Trump would require the votes of 20 Republican senators (out of 53), assuming all Democrats and independents voted in favor. That’s a high hurdle, but Trump’s support among Republican senators has always been guarded, not to say grudging. Indeed, just count the GOP senators whom the president has publicly insulted or gotten crosswise with at some point and you have nearly half the GOP votes needed for removal. In that sense, Trump is potentially more vulnerable to impeachment and removal than most presidents would be, which should reduce the burden for Democrats.

And yet the Democrats have failed to carry even this reduced burden. Because Mueller noted there could have been a legitimate motive for most of the possible instances of obstruction, his report left Democrats in the position of having to prove that any benefit of these acts to the president was not merely incidental, but was in fact the purpose of the acts. Faced with that daunting evidentiary challenge, they searched for more promising grounds for impeachment.

No such luck: The grounds they alighted on raise the same evidentiary challenge. Did the president condition aid to Ukraine and a White House meeting with President Volodymyr Zelensky exclusively on a favor that would benefit him personally? Or was the personal political benefit merely incidental to a legitimate government purpose, as happens all the time in every presidency? After all, President Barack Obama’s “hot mic” conversation with Russian President Dmitry Medvedev asking him to “take it easy” on arms control before the 2012 election, and promising a more accommodating U.S. stance toward Russia afterward, was not impeachable, though he was explicitly asking for something that would at least incidentally benefit him in the upcoming election.