Speculation about Special Counsel Robert Mueller’s investigation has turned toward obstruction of justice—specifically, whether President Trump can be criminally prosecuted for firing James Comey as director of the Federal Bureau of Investigation or for earlier asking Mr. Comey to go easy on onetime national security adviser Mike Flynn. The answer is no. The Constitution forbids Congress to criminalize such conduct by a president, and applying existing statutes in such a manner would violate the separation of powers.

The Constitution...