If Dershowitz is right, even the articles of impeachment that were drafted against President Richard Nixon would have been constitutionally defective. Nixon was not the first president to try to use the power at his command to investigate and spy on his political enemies, but the Watergate scandal was part of the process by which Americans insisted presidents should never take such actions again. The ignominious end of the Nixon administration stood as a warning to presidents not to attempt such abuses of power in the future.
If Republicans in the Senate were not only to acquit Trump but also to endorse the constitutional argument that Trump’s lawyers made on his behalf, they would be taking a large step toward undoing the lessons of Watergate. They would be inviting future presidents to think that there is only one possible check on the abuse of power, and that is the ballot box. They would be crippling the ability of future Congresses to hold presidents accountable for abuse of power and to deter presidents from running the risk of facing an impeachment inquiry for their misconduct while in office.
So when senators do vote to acquit, they must take the opportunity to make plain that presidential abuse of office is still intolerable.