The Dershowitz argument as I understand it is that abuse of power alone is not impeachable. That is because it is a vague catchall that many Presidents have been accused of. The Obama “I’ve got a pen" doctrine is arguably a philosophy for abusing Presidential power. You can go back to Lincoln and Jackson and Jefferson, all accused of abuse of power. Few considered their abuses of power to be impeachable. So that raises the question of what types of abuses of power might be impeachable. Nixon’s impeachment Article 2 was for abuse of power. The specific charges included ordering the IRS to audit political opponents and having the FBI conduct surveillance having nothing to do with legitimate law enforcement.
Notice that none of those abuses of power depend on Nixon’s state of mind. But they are abuses of power that rise to the level of high crimes.
Now Trump is guilty of pausing the delivery of aid for a few weeks. The pause was lifted before the statutory deadline. The goal may or may not have been to pressure Ukraine to dig up dirt on a political rival. Or it may have been to get Ukraine to investigate possible corruption by the Bidens. There is no way of proving what was really in his mind. In any event, in the key phone call, Trump never made an explicit threat and never tied aid to an investigation. The Ukrainians deny they felt threatened or pressured. So where is the impeachable abuse of power? As to the “do us a favor” request to investigate the 2016 election, it is not clear how that is even improper under any interpretation. Oh yes, it violates that non-existent law that forbids investigating a story some in the media claim is debunked.
So let’s summarize: ordering the IRS to audit your political enemies is impeachable abuse of power, pausing foreign aid for a short time is not.