Republicans are trying to argue that either:
The POTUS is entitled to define U.S policy
The extortion of Ukraine did not work
The first is consistent with Republican preference for the unitary executive relieving the Congress of all responsibility and the second is akin to arguing that attempted crimes should not be of concern.
The central problem, of course, is that extortion is SOP on Capitol Hill. If Trump is guilty, then so are a lot of Congressmen.