Here is Lamar Alexander’s argument for acquittal, slightly embellished to add clarity:
1. Yes, the overwhelming evidence shows that Trump attempted to coerce a foreign government to assist with his re-election campaign in a way that was inconsistent with American law and national interests.
2. Yes, that is an abuse of power, and impeachment was consequently appropriate.
3. The Senate, however, is required to determine whether the offenses committed by Trump represent such a clear and present danger to the country that he should be removed from power.
4. That is a balancing test. In this case, the attempted coercion of Ukraine failed. In addition, the election is imminent, and Trump’s fanatical supporters are capable of doing just about anything if he is removed. The danger to the country from removal, based on these unique circumstances, is greater than the danger from keeping him in office. Therefore, Trump did not commit a “high crime and misdemeanor,” as that term is used in the Constitution.
There is nothing in the text of the Constitution itself which suggests that such a balancing test is appropriate. Since the meaning of “high crimes and misdemeanors” is left open to events and interpretation, however, you can reasonably argue that there is enough wiggle room for this kind of approach.
The real danger arising from the balancing test is that it will encourage future corrupt presidents to do what Trump does–play exclusively to the base and threaten violence if he is deprived of power. Would Alexander do the balancing the same way if he had to deal with lightly armed Bernie bros, as opposed to Trump supporters? I’m guessing not.