Harvard Law professor Alan Dershowitz on Monday said President Trump was within his rights as commander in chief when he fired former FBI director James Comey, and warned Democrats trying to take him down on obstruction of justice charges that they won't succeed.

"You cannot charge a president with obstruction of justice for exercising his constitutional power to fire Comey and his constitutional authority to tell the Justice Department who to investigate, who not to investigate. That's what Thomas Jefferson did, that's what Lincoln did, that's what Roosevelt did. We have precedents that clearly establish that," Dershowitz told Fox News' "Fox and Friends" Monday morning.

The issue came up Monday after Trump's lawyer, John Dowd, said the president cannot obstruct justice. One of Trump's tweets over the weekend suggested to some that Trump knew his former national security adviser, Michael Flynn, lied to the FBI, which could imply obstruction of justice.

On Sunday, Senate Judiciary Committee ranking Democrat Dianne Feinstein said she is part of a group creating a case of obstruction of justice against Trump.

Dershowitz warned that in order to go after a president for obstruction of justice, "clearly illegal acts" would have had to have been committed."

Even in the case of former President Bill Clinton, who influenced potential witnesses not to tell the truth, there was no obstruction of justice charges ever seriously considered, the liberal-affiliated attorney added.