Alan Dershowitz, professor emeritus at Harvard Law School, argued Monday that Trump should not face any criminal penalty just because he fired former FBI Director James Comey.

"The president of the United States should not be subject to criminal prosecution for merely exercising his constitutional authority," he said on CNN.

"In the absence of any statute to the contrary, the president has the authority fire the director of the FBI, and the president has the power tell the director of the FBI who to investigate, who not to investigate," he added.

Democrats have argued for months now that Trump fired Comey to disrupt the FBI's investigation into Russia's meddling in the election, which Democrats also say could show that Russia was colluding with Trump to bring down Clinton. Democrats admit there is no evidence of collusion so far, and recently have been arguing that Trump obstructed justice by encouraging Comey to drop the case.

But while Democrats have been saying Trump is trying to cover his tracks the way President Richard Nixon did, Dershowitz said the case is nothing like the Nixon coverup.

"This is not the Nixon case. This is the Bush case," he said, referring to President George H.W. Bush's pardon of former Defense Secretary Casper Weinberger. Weinberger was thought to have information linking Bush to the Iran-Contra affair.

"Nobody suggested obstruction of justice" in that case, Dershowitz said.

Dershowitz added it makes no sense to expand obstruction of justice to cover "constitutionally authorized" actions by the president.