Perhaps this question is asking for speculation, but I am interested in knowing what is known about this matter: Alan Dershowitz is a noted Civil Libertarian. How did he end up defending President Trump?
This Salon article is written in quite acerbic manner, but it seems that Dershowitz being a defender of Trump on TV was a factor in his selection:
And then there are the Fox News "stars," meaning Ken Starr, Alan Dershowitz, Robert Ray and Pam Bondi, who were obviously brought on because Trump wanted to hire the lawyers from "Law and Order" but was gently informed that they were fictional characters. (OK, I made that part up — but it's sad that such a thing is so believable.) He settled for Fox News talking heads who can speak credibly to the only people who matter — his base.
So, assuming this correct, the question becomes: why was Dershowitz defending Trump on TV, before officially becoming his lawyer. A part of the answer seems to be that Dershowitz's take on what is impeachable is setting quite high bar (and this is quite useful to Trump):
Alan Dershowitz, the celebrity lawyer and Harvard professor emeritus named to President Donald Trump’s legal team last week, is already facing questions about his defense of the president in light of comments he made 21 years ago about impeachment.
Appearing on “This Week with George Stephanopolous” on Sunday, Dershowitz argued that impeachment of a president requires proof “of an actual crime,” and that Trump can’t be removed for abuse of power or obstruction of Congress, the two impeachment articles approved by the House last year.
“It needn't be a statutory crime, but it has to be criminal behavior, criminal in nature,” he said. [...]
But Dershowitz appeared to make a very different argument in 1998 when he appeared on CNN’s "Larry King Live" and said impeachment didn’t require the president to commit a crime.
"It certainly doesn't have to be a crime if you have somebody who completely corrupts the office of president and who abuses trust and who poses great danger to our liberty, you don't need a technical crime,” he said in comments unearthed by CNN. [...]
Dershowitz, a self-described liberal Democrat who said he voted for Hillary Clinton in 2016, opposed President Bill Clinton’s impeachment, and wrote a book on the subject in 1998. He also consulted with Clinton's legal team during his impeachment.
Pressed on his decades-old comments in an MSNBC appearance on Monday, Dershowitz said he was making the "same argument."
“It doesn’t have to be technical crime because at the time the framers wrote the Constitution, there was no criminal code.”
Dershowitz echoed that in a statement to ABC News, saying, "That’s still my position. It has to be criminal -- like, akin to treason or bribery. Not abuse or obstruction."
On his role defending Trump, Dershowitz said he has a "limited role" in the Senate trial.
"I’m only in the case as of counsel on the constitutional criteria for impeachment. I’m not involved in the strategic decisions about witnesses or fact," he said.
Dershowitz also wrote a book published in 2018 titled The Case Against Impeaching Trump. Trump apparently liked the book back then and tweeted about it... despite (or perhaps because?) of the book's somewhat unorthodox nature; quoting a WaPo review...
In the past year, three books have made the case that President Trump has committed offenses so dire that he might constitutionally be impeached and removed from office. The authors differ in how they approach the question: Allan J. Lichtman (“The Case for Impeachment”) tackles it head-on; Laurence Tribe and Joshua Matz (“To End a Presidency”) build a case against the president but caution against moving too hastily; Cass Sunstein (“Impeachment: A Citizen’s Guide”) weighs a range of hypotheticals but fastidiously refrains from mentioning Trump’s name.
Now comes Alan Dershowitz with his book “The Case Against Impeaching Trump.” From the title, the reasonable reader might expect the former law professor — whose television appearances defending the president, or at least criticizing his detractors, have become increasingly unavoidable — to present a sustained response to the arguments of his fellow authors.
This is true for roughly the first 30 pages. The rest of the 150-page book is bulked out by copies of Dershowitz’s op-eds and columns, transcripts of his various television appearances, and — strangest of all — a series of his tweets. The result is an odd jumble that is less a legal argument against impeaching Trump and more a scrapbook of what Dershowitz has been up to for the past year.
Dershowitz has lightly edited his short writings in the interest of linking them together into a volume. He refers to them here as “essays,” which is something of an overstatement: Most are no more than two or three pages in length. (By my count, only one of the 28 pieces, in addition to the introduction, appears to have been written originally for this book.) Relatively few of them are about impeachment per se, and some have nothing to do with the Russia investigation at all. One, first published as a Wall Street Journal op-ed, was written in response to Trump’s comments on the violence in Charlottesville.
Some chapters involve reflections on evergreen topics — Dershowitz’s desire for a nonpartisan congressional commission to investigate election interference, for example, and his belief that civil libertarians (except him) have sold out their ideals because they want to bring down the president. [...]
A chapter on why Dershowitz distrusts allegations of Trump’s “corrupt motive” in dismissing FBI director James Comey — something prosecutors would need to show in bringing an obstruction case — somehow transforms into a list of angry emails he has received for his defense of the president. The overall impression is that Dershowitz understands his own martyrdom (yes, there is mention of Martha’s Vineyard, where, Dershowitz has argued, his friends now snub him) as a proxy for the supposed injustices of the Mueller investigation.
The review then details Dershowitz’s standard for impeachment (which I won't quote against because it's the same as in the more recent quotes), but it does have this (final) nugget:
To Dershowitz’s credit, he recognizes the bizarre outcomes his position would generate: Even if a hypothetical president chose to stand by while Vladimir Putin reclaimed Alaska as Russian territory, Dershowitz argues, that president could not be impeached.
As an aside, it's perhaps interesting to note that Dershowitz has also defended Netanyahu against allegations of corruption on somewhat similar terms, i.e. that there's prosecutorial overreach:
Israeli Prime Minister Benjamin Netanyahu will be indicted, pending a hearing, for three alleged acts of corruption, according to Israel’s attorney general. But are the “crimes” at issue really crimes? Has the Israeli legislature — the Knesset — actually criminalized the conduct the prime minister is charged with committing? If not, would the Knesset ever pass statutes to criminalize such conduct? Is it proper for prosecutors to stretch existing laws to fit his conduct? [...]
In fact, he even makes a comparison:
The larger point is that Israel has vague, elastic and open-ended criminal laws that can be stretched to target political opponents. So does the United States. In our country, such laws include obstruction of justice, conspiracy and witness tampering, as we have seen in the ongoing investigations of President Trump.
Professor Dershowitz made the answer clear in this interview, where he said:
Look, I'm in this process because I love my country and I love the Constitution and I don't want to see the Constitution weaponized into a political tactic that can be used against any president.
Professor Dershowitz agreed with an interviewer's comment that political partisanship has weakened the case for both sides, saying:
Absolutely. It's become too partisan. You know it was congressman Nadler who said "You don't impeach unless there is broad bipartisan support." Hamilton said "The greatest danger is that impeachment will turn on the number of votes and the party who has the most votes in each house." That great danger has come about.
You should never have an impeachment unless there is broad, widespread, nonpartisan support. That doesn't exist here.
Dershowitz loves the public attention of a big, usually indefensible case.
This fits his MO perfectly: Lots of TV exposure where he can say outrageous things (like the Alaska claim).
Unfortunately, his arguments just aren’t true. And he’s trying to sew doubt to get the benefit of it.
But he’s not presenting to a group relatively light in legal knowledge. The jury in this case is chock-ablock with lawyers. So appealing to emotion masquerading as principle, and cutting full interpretations out of whole cloth isnt going to sway anybody. If anything it makes Trump look reckless, defensive, and poorly educated. Dershowitz is failing one of the key rules of trial law: try the case in front of you and stay away from abstract and hypothetical cases.
Also, recall that Dershowitz is not a constitutional scholar. He’s more a criminal law attorney. He has a colleague from Harvard - Lawrence Tribe - who is a constitutional expert. I would love to hear more from him.