Thanks for the link to ZH instead of CNN.

From the link:

STATEMENT REGARDING PROFESSOR DERSHOWITZ’S ROLE IN THE SENATE TRIAL -

Professor Dershowitz will present oral arguments at the Senate trial to address the constitutional arguments against impeachment and removal. (1of 2)

(2 of 3) While Professor Dershowitz is non partisan when it comes to the constitution—he opposed the impeachment of President Bill Clinton and voted for Hillary Clinton— he believes the issues at stake go to the heart of our enduring Constitution.

(3of 3) He is participating in this impeachment trial to defend the integrity of the Constitution and to prevent the creation of a dangerous constitutional precedent.

Alan Dershowitz is a Liberal, and competent, Constitutional scholar, who has already written on the topic. We already know what he thinks.

He would not be a fact witness, but a legal/historical witness. His testimony would be knowledgeable and respectful to the Constitution, and therefore run in Trump's favor.

See for example, the article linked below.

https://thedonald.win/p/3nLNwwD/

https://www.newsmax.com/alandershowitz/dershowitz-trump-impeachment-pelosi/2019/12/05/id/944720/#ixzz67L8XI4vB

and his comments during the "proceeding," which support another competent lawyer, Johnathan Turley.

https://www.washingtonexaminer.com/news/alan-dershowitz-shoots-down-impeachment-witness-claims

It is fundamental to the safeguarding of our laws that scum like Epstein are allowed legal representation. Representing a person is not the same thing as participating in their crimes.