The president may not be prosecuted in a criminal judicial proceeding for exercising his discretion, however objectionably, in executive matters over which the courts have no power of review. If Mueller tried to indict him, Trump would have unfettered discretion to fire Mueller and to direct the Justice Department to drop the case. You may not like that, but that’s the way it is. It is not, however, the end of the matter. Any powers can be abused. When executive powers are abused, Congress retains the constitutional authority to impeach and remove the president. Obstruction of an FBI investigation may...