Harvard law professor emeritus Alan Dershowitz compared special counsel Robert Mueller’s lack of conclusion on whether President Trump obstructed justice to former FBI Director James Comey's handling of the investigation into Hillary Clinton's use of an unauthorized, private email server.

"Shame on Mueller for not having the guts to come to a decision one way or the other,” Dershowitz told Fox News on Sunday hours after Attorney General William Barr sent a four-page letter to Congress with an analysis of Mueller's final report.

“The job of the prosecutor is to decide, yes or no. Make a decision. And then if you say yes, you indict. If you say no, you shut up. You don't go on and say, no, we're not going to indict, but let me tell you all the evidence that might have led us to indict. That's exactly what prosecutors shouldn't do,” he said. “That's exactly what Comey did. How is this different than Comey? Comey says I'm not going to indict Hillary Clinton, but let me tell you it was a close case.”

Dershowitz, who has been a vocal defender of Trump in matters related to the Mueller investigation, was referring to a controversial press conference in the summer of 2016 during which Comey announced he would not recommend charges against Clinton.

Barr's four-page letter said the 22-month investigation did not find evidence of collusion between the Trump campaign and Russia. Barr also quotes Mueller saying “while this report does not conclude that the President committed a crime, it also does not exonerate him," after which the attorney general said he and Deputy Attorney General Rod Rosenstein found there was a lack of “sufficient” evidence to determine whether Trump obstructed justice.

Trump told reporters Sunday he had been completely vindicated by the report. Meanwhile Democrats unsatisfied with the letter are calling for the release of full Mueller report and House Judiciary Chairman Jerry Nadler, D-N.Y., vowed to bring Barr in to testify before Congress "[i]n light of the very concerning discrepancies and final decision making at the Justice Department."

Dershowitz said Mueller’s failure to say whether or not Trump obstructed justice will be cast shadow over the president.

“If there's an indictment, at least you have a chance to challenge it and fight back in court,” he said. “But if they say, ‘Oh, you were a bad boy, you almost obstructed justice,” how do you fight against that? Where do you defend yourself against that kind of charge?”