Not sure how it’s bad for liberty if the executive branch investigates itself or bogs itself down a little. In other recent interviews, Dershowitz seems to support the imperial Presidency.
I think Dershowitz is simply being honest and professional here. Toobin is insane to suggest that Trump saying "Go Wikileaks" should be the basis for a criminal investigation. Obama overthrew the elected government of Ukraine and installed Nazis in its place. The ethnic Russian population of Crimea was appalled and voted to rejoin Russia a few weeks later. That popular move to rejoin Russia was the alleged basis of Obama's sanctions on Russia. All Trump ever did was suggest it would be wise to talk things over with Russia and Putin. For whatever reason, the powers-that-be cannot tolerate such a move, hence the Russia Russia Russia B.S. And it continues even though Trump is doing basically what Obama did and what Hillary would have done in continuing the insane levels of hostility toward Russia.