"Quid pro quo" is not only not illegal, it is the essence of affairs between States that do not involve warfare. When FDR traded destroyers for bases in the Caribbean, should he have been impeached?
Thats a terrible defense if there is no truth to it.
The Trump defense team has done an excellent job to date making two critical points: (1) There is no truth to the allegations; and (2) Even if the underlying charge was true, theres nothing wrong with it.
They did a very effective job on Item (2) not by making the case that the President had the authority to impose conditions on U.S. aid to Ukraine, but by stressing that the President has an obligation to ensure that U.S. taxpayer money isnt wasted on corrupt undertakings overseas.