That is, when it comes to Trump, the episodes prosecutors have been investigating as obstruction do not involve criminal actions — such as tampering with witnesses, as was done in the Nixon and Clinton impeachment cases. Instead, they involve a president lawfully exercising his constitutional prerogatives, but driven by what prosecutors portray as improper motives. That would be a very controversial theory of obstruction. As William Barr opined in a memorandum written before he became attorney general, a president should not be indicted over conduct in a gray area — it should be only over misconduct that is clearly criminal and serious.

Impeachment, however, is saliently different from indictment. Unlike a prosecutor, Congress does not need a penal offense to take action; impeachment is a political process (i.e., the stripping of the president’s political authority by the other political branch). It can be driven by misconduct that Congress concludes is an abuse of power, even if it does not constitute an offense of the criminal code.

I do not expect collusion to be the highlight of Mueller’s report. Collusion was just the rationale for conducting an investigation for which there was no criminal predicate. I expect Mueller to file a report that highlights obstructive conduct, though probably not one that calls for an obstruction indictment. That, as I said back in December 2017, would throw the ball into Congress’s court for consideration of impeachment.