Presidential actions in the public interest

Harvard law professor emeritus Alan Dershowitz has given a flawless defense of President Trump, and almost no one disputes him directly. They either argue that he changed his opinion from 20 years ago, or they distort what he said about actions in the public interest.

He admits that his opinion has shifted slightly from 20 years ago, based on research into Andrew Johnson's impeachment. His opinion has changed very little.

The latter attack are against this Dershowitz speech, in answer to a question:

The only thing that would make the quid pro quo unlawful is if the quo were in some way illegal. Now, we talked about motives.  There are three possible motives a public official might have.  The first is in the public interest…. The second is in his own political interest. And the third would be in his own financial interest, just putting money in the bank….

I want to focus on the second. Every public official believes that his election is in the public interest…. And if a president does something that he believes will help him get elected, in the public interest, that cannot be the kind of quid pro quo that results in impeachment.

I think the problem here is the dangling phrase "in the public interest", in the last sentence.
he
Based on the obvious meaning, and on his explanations, I think Dershowitz meant this: If the president does something in the public interest, then he cannot be impeached for believing that it will help him get elected.

That is, "in the public interest" modifies "does something", not "he believes".

This argument may have been convincing to a lot of Senators. They all do things to get elected all the time. They can justify them by saying that they are in the public interest. Ideally they are doing things that are both in the public interest and helping to get elected. This is the nature of politics. The President is allowed to do the same.

A comment says:

The first question Dersh asks is, “Was the act itself criminal?” (Or very crime-like, anyway.) If the answer is “yes”, we need go no further, it’s impeachable. Good motives can’t redeem objectively criminal acts.

If the answer is, “no”, let me quote Dersh: “the president does something legal completely within his power, but he was motivated in part by a desire to get reelected, would that turn that motive into a corrupt motive?”. ...

What he’s saying here is that having a political motive for doing something doesn’t taint an act which could otherwise be justifiable.

That is correct.