The risks in the approach are both real and substantial. But after having served as a prosecutor in the Department of Justice, as a senior counsel in the Whitewater investigation of Clinton, and as a Bush appointee at the Department of Homeland Security, I’ve come to recognize that challenging, balanced judgments of the sort necessary today are sometimes forced on us by circumstances beyond our control. Hard choices do, sometimes, make bad law, but they cannot always be avoided. To decline to investigate Trump’s alleged criminality after he has left office is itself a choice—and it’s the wrong one.