The four threats to Robert Mueller

Issuing pardons: The president may also try to pardon away the special counsel’s investigation. Mr. Trump could grant pardons to individuals who have already pleaded guilty, such as Michael Flynn, his former national security adviser, who pleaded guilty to lying to the F.B.I. this month. Or Mr. Trump might do so prospectively, to those who may be targets, such as his son-in-law, Jared Kushner, or his son Donald Trump Jr. Mr. Trump might even issue one to himself (he reportedly asked about it), whether individually or as part of a blanket pardon of all those involved in the Russia investigation.

Here, too, voices must be raised now, and loudly, with Senator Mark Warner’s strong comments this week in defense of Mr. Mueller as an outstanding first step. Congress has a particularly important role to play, making clear that such pardons would be a red line that would lead to abuse-of-power hearings in the House Judiciary Committee and possibly impeachment.

That outrage is not just a matter of political or moral sentiment — Mr. Trump would be running serious legal risks with this course of action. Pardons made to family members with corrupt intent might (like the president’s firing of James Comey, the former F.B.I. director) constitute obstruction of justice, opening him to additional federal investigation. Self-pardon is contrary to principles of the rule of law that are embedded in our Constitution and might be judicially rejected. And the president cannot pardon state-level crimes, which multiple state attorneys general and prosecutors are investigating.