Hmm. Every sentence of Dershowitz's argument is a lie. Every one.

Dersh's argument.

It's neither sound nor even formally valid.

Premise: All politicians believe their own electoral victories are in the public interest. 

Premise (or aside?): Mostly, they are right. 

Conclusion: So if the quid pro quo is help getting elected, that cannot be the kind of quid pro quo that justifies removal from office.