A dangerous argument is now being put forward by some Democratic ideologues: namely that President Trump should be indicted for the crime of obstructing justice because he fired FBI Director James Comey. Whatever one may think of the president's decision to fire Comey as a matter of policy, there is no legitimate basis for concluding that the president engaged in a crime by exercising his statutory and constitutional authority to fire director Comey. As Comey himself wrote in his letter to the FBI, no one should doubt the authority of the president to fire the director for any reason or no reason.

It should not be a crime for a public official, whether the president or anyone else, to exercise his or her statutory and constitutional authority to hire or fire another public official. For something to be a crime there must be both an actus reus and mens rea – that is, a criminal act accompanied by a criminal state of mind.

Even assuming that Trump was improperly motivated in firing Comey, motive alone should never constitute a crime. There should have to be an unlawful act. And exercising constitutional and statutory power should not constitute the actus reus of a crime. Otherwise the crime would place the defendant's thoughts on trial, rather than his actions.

Civil libertarians, and all who care about due process and justice, should be concerned about the broad scope of the statute that criminalizes "obstruction of justice." Some courts have wrongly interpreted this accordion-like law so broadly as to encompass a mixture of lawful and unlawful acts. It is dangerous and wrong to criminalize lawful behavior because it may have been motivated by evil thoughts. People who care about the rule of law, regardless of how they feel about Trump, should not be advocating a broadening of obstruction of justice to include the lawful presidential act of firing the FBI director. Such an open-ended precedent could be used in the future to curtail the liberties of all Americans.

So let's put this nonsense behind us and not criminalize policy differences, as extremists in both parties have tried to do. Republican and Democratic partisans often resort to the criminal law as a way of demonizing their political enemies. "Lock her up," was the cry of Republican partisans against Hillary Clinton regarding her misuse of her email server. Now "obstruction of justice" is the "lock him up" cry of partisan Democrats who disagree with Trump's decision to fire Comey.

I opposed the criminalization of policy differences when Texas Governor Rick Perry, Congressman Tom Delay and Senator Bob Menendez were indicted, and I strongly oppose the investigation now being conducted against Israeli Prime Minister Benjamin Netanyahu. The criminal law should be used as the last resort against elected officials, not as the opening salvo in a political knife-fight. There is no place in a democracy for elastic statutes that can be stretched to fit lawful conduct with which political opponents disagree. If they are allowed to be stretched today to cover your political enemies, they could be stretched tomorrow to go after your political friends.

The debate over the propriety of the president's actions, about which I have opined repeatedly, should continue but let's take the allegations of criminal obstruction of justice out of this important debate. There is more than enough fodder for a debate over the merits and demerits of the president's actions without muddying the waters with politically-motivated charges of criminality.

Partisanship seems to have no limits these days. Both parties are equally at fault, as are extremists among the public and within the media. It is getting harder and harder to have a nuanced debate about complex political issues. Everything is either evil or good. Nothing has elements of both. Actions either deserve criminal indictment or the Nobel Prize.

Nobody benefits from this kind of ideological shouting match. So let's agree to disagree about important issues, but let's not distort the debate with extremist slogans like "lock her up" or "obstruction of justice." We are better than that.

Alan Dershowitz (@AlanDersh) is the Felix Frankfurter Professor of Law, Emeritus, at Harvard Law School and author of "Taking the Stand: My Life in the Law" and "Electile Dysfunction: A Guide for the Unaroused Voter."

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