And, yes, there are political difficulties too. The “Lock Her Up!” chants at 2016 Trump rallies against Hillary Clinton for her use of a personal email server while Ms. Clinton was secretary of state were so pernicious because threatening to jail political enemies can lead to a deterioration of democratic values. If each presidential administration is investigating and prosecuting the last, respect for both the electoral process and legal process may be undermined.

That concern is real, but if there has ever been a case extreme enough to warrant indicting a president, then this is the case, and Mr. Trump is the person. This is not just because of what he will do if he is elected again after not being indicted (and after not being convicted following a pair of impeachments, one for the very conduct under discussion), but also because of the message it sends the future.

Leaving Mr. Trump unprosecuted would be saying it was fine to call federal, state and local officials, including many who have sworn constitutional oaths, and ask or even demand of them that they do his personal and political bidding.