President Trump's impeachment trial in the Senate begins tomorrow. One of the lawyers representing the president is Harvard law professor Alan Dershowitz. Dershowitz spoke with WGBH News' All Things Considered anchor Arun Rath
about his role on Trump's legal team. This transcript has been edited for clarity.

Alan Dershowitz: I'm a special counsel on the constitutional issues regarding impeachment. I have a limited but important role: as somebody analogized, it's like I'm playing special teams in the Super Bowl. I've been asked to perform a specific task. Namely to research and describe and outline the constitutional arguments as to why the two allegations against President Trump — abuse of power and obstruction of Congress — don't rise to the level of impeachable offenses under the Constitution, and I'm working on preparing my argument.

Arun Rath: This, though, is big. As someone who's been a professor of constitutional law, to speak about this before the Senate, are you excited? What what are your feelings going into it?

Dershowitz: Well, this is very important. I'm making an argument that I think applies not only to this president but every president. Namely, that you can't be impeached and removed for abuse of power. Half of our presidents have been accused by political opponents of abusing their power. That's not one of the criteria that the framers settled on after they discussed a variety of criteria. So, yeah, I'm excited to be able to contribute to the national dialogue. What's been disappointing to me is that since my appointment was announced most of the discussion has been ad hominem attacks on me and others on the team, and not getting to the substance. So, I'm glad to have an opportunity to discuss the substance of my arguments with you.

Rath: Well, let's talk about the substance more, and we'll break this down. You started talking about abuse of power. Are there any circumstances under which abuse of power could be an impeachable offense?

Dershowitz: No. You can be charged with an impeachable offense which constitutes an abuse of power but the opposite isn't true. You can't just be charged with an abuse of power. That's like saying you're being charged with being a bad president. Is there anything that a bad president can do that would result in impeachment? Sure. He could commit bribery or treason, but abuse of power alone is not a basis. And so since the allegations, abuse of power and obstruction of Congress, don't satisfy the Constitution, that's really all the Senate has to decide.

Rath: In terms of the details of the abuse of power, we heard recently that the nonpartisan Government Accountability Office says that the president did break the law. If that is part of the abuse of power, assuming that is the case, that wouldn't rise to the level of being impeachable?

Dershowitz: First of all, the GAO was totally wrong. They've got it backwards. What they said is that the president has no authority to substitute his priorities for those of Congress. It's exactly the opposite. When it comes to foreign policy, Congress has no authority to substitute its priorities for those of the president. To give an example, let's assume a liberal Congress decided to give a billion dollars to Cuba. The president said, no, that's not my foreign policy. I don't support Cuba, and I'm not going to give the billion dollars. That would be within his authority. And if the statute said no, the statute would be unconstitutional. So the GAO got it exactly backwards and wrong. And they often do, because they're a branch of Congress, basically, and they always favor Congress over the president. And that's to be expected. They are a congressional organization. They're nonpartisan between Democrats and Republicans, but they're very supportive of Congress's power over that of the president.

Rath: Your argument regarding obstruction of Congress, is it the same approach? That this is not something that is defined as impeachable?

Dershowitz: That's even worse. There's no such thing as obstruction of Congress. It's made up. There's nothing in any law about it, there's nothing in the Constitution about it. It's similar to what Andrew Johnson was impeached for. I think Article 10 of his impeachment said he demeaned Congress, he insulted Congress. That's political. The president simply said that before members of the executive branch will submit to what he believed were partisan subpoenas, that they had to get court enforcement, go to the court and enforce it. And if the courts were to do it, of course, the president will obey. But, under Federalist No. 78 by Alexander Hamilton, when there's a conflict between the legislative and executive branches, the legislative branch doesn't just win. It has to go to the courts, and the court is the umpire. And that's what the Democrats refused to do because they were rushing to impeach and then they delayed it a month, and now they're calling for witnesses, which would delay it even more because many of the witnesses would be challenged under executive privilege. So, no, obstruction of Congress is made up.

Rath: Democrats have charged that taking these actions in aggregate, the president has abandoned — that word has been used — abandoned his oath of office and ...

Dershowitz: You know, they should have voted on that. If they thought that that was the criteria, they should have said, article three is violation of the oath of office. That also would not be an impeachable offense. But if they could say article four was treason or article five was bribery, that would change everything. But the only two articles they voted on were abuse of power and obstruction of Congress. And that's what the Senate has to vote on, not anything else.

Rath: When we last spoke, we were speaking with you just as a legal expert. You were not part of the president's team. When did they reach out?

Dershowitz: Well, it's been a process, and an interesting process, because they asked me for a while and my wife was very much against it.

Rath: Because it fuels the type of attacks you've been hearing?

Dershowitz: Well, no, she's used to the attacks I've been getting. But she thought it would be better if I remained independent and could make the positions I'm making as an independent person. Obviously, once I'm asked to present his argument in the Senate, I no longer have that degree of independence, although I still think that I'm going to make an argument that's an independent argument. But my wife was opposed to it. The president spoke to my wife. We happened to meet by coincidence, we were having dinner with friends of ours at Mar-a-Lago on a night that the president was there. He came over to my wife and talked. She expressed her views about my independence, he expressed his views. And then he said, Alan, Carolyn, do whatever you think is right. Then he called me a few times and we firmed it up basically last week.

Rath: You have to understand, I've got to ask a question like this: you can't talk about your conversations in detail, but what what is your relationship like with your client, the president of the United States?

Dershowitz: Well, I've only met him seven or eight times in my life. We're not acquaintances or friends. I voted against him in the last election. I have an open mind as to the next election and I don't generally advertise in advance who I'm voting for unless I endorse a candidate. So my relationship with him is cordial but lawyer-client, at this point.

Rath: You've represented a lot of different clients. I'm just wondering how the president compares to other clients.

Dershowitz: Well, no two clients are the same. I mean, O.J. Simpson tried to tell me how to argue the case in every respect and he insisted on taking the witness stand and all of that. Other clients, like Mike Tyson, left it in my hands completely. Different clients play different roles. President Trump is the president of the United States and therefore, he's concerned not only about the legal issues but obviously the political implications. I try very hard not to talk about any of those. He knows that I'm a liberal Democrat. He doesn't like that, but he wants my talent or my research. And I'm the only person on the "team" that strongly opposed Clinton's impeachment and is a liberal Democrat and whether there is virtue to that, I don't know. But I was asked to do it, and the president was fully aware, obviously, of my political affiliations and my political preferences.

Rath: We've heard rumblings that there's a possibility that some of the president's legal issues might follow him out of office. If Donald Trump were to, once he's out of office as a private citizen, ask for your legal representation, would you work with him again?

Dershowitz: Well, if it were constitutional issues, I might. I usually represent people only once. I don't think I've ever represented somebody twice. I know I didn't want to represent O.J. Simpson when he was arrested a second time. This is different. If it were constitutional issues, I would certainly consider it.