The left is borrowing tactics from the old Soviet Union in an attempt to bring down President Trump, contends Harvard law professor Alan Dershowitz.

In an interview with "Fox & Friends," Dershowitz said the charge that Trump is mentally unstable and should be removed from office is similar to what communist leaders during the Cold War did to political dissidents he represented.

"There's only one thing worse than trying to criminalize political differences, and that's trying to psychiatrize them," said Dershowitz, the Washington Examiner reported.

The charge of mental incompetence is a major theme of a book by journalist Michael Wolff, "Fire and Fury: Inside the Trump White House," that has consumed the news cycle since excerpts were released last Wednesday.

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The White House has called the book, which also claims Trump didn't want to win the election, "trashy tabloid fiction" and pointed out that a number of subjects dispute quotes attributed to them, including a Trump friend and adviser quoted calling the president "stupid."

Wolff admitted Monday in an interview with "CBS This Morning" that he did not interview Vice President Mike Pence or any Cabinet members, even though some of the most provocative claims in his book are credited to Cabinet members, the Daily Caller reported.

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Dershowitz told Fox the "psychiatrists now who are trying to diagnose without ever having met the man, that's what they did in Russia."

"I represented dissidents who they locked up in mental hospitals," he said. "That's what they did in China. That's what they did in apartheid South Africa.

"How dare liberals, people on the left, try to undo democracy by accusing a president of being mentally ill without any basis," he said.

Wolff said in an interview Sunday with "Meet the Press" that concern in the White House about Trump's fitness for office has become so great that members of the administration routinely talk about a constitutional solution.

The reference is to the 25th Amendment, which says a majority of the Cabinet can decide to remove the president once he or she is "unable to discharge the powers and duties of his office."

Last June, Rep. Sheila Jackson Lee, D-Texas, urged Congress to use the 25th Amendment to remove Trump from office, arguing "it is utilized when a president is perceived to be incompetent or unable to do his or her job."

She cited Trump's failure to act quickly enough to counteract Russia's election meddling as an example of incompetence. Legal experts, however, warned against using the amendment to resolve political disputes.

Under the Soviet Union, the KGB designed a system to use psychiatry against political opponents, presuming anyone who opposes the communist system must be mentally diseased or incompetent.

In a 1959 speech published in the Communist Party newspaper Pravda, Premier Nikita Khrushchev said: