There’s nothing in the law specifying circumstances under which it would be illegal for the president to fire the FBI Director. It just says the president can. That’s why the legal case for obstruction is weak.

You might not buy that. And there are some lawyers who agree with you (though there are also many who do not). Regardless, considering the balance of the Supreme Court and who controls the Justice Department, the argument that the president cannot be indicted — especially not for conducting constitutionally designated duties — is likely to hold sway.

Congress, however, can consider the president’s motivation, and determine that firings in bad faith violate the Take Care clause. That’s why the impeachment case is stronger, and why someone looking to prosecute President Trump for obstruction would be smarter to follow the impeachment, rather than criminal, track.