He graduated top of his class from Yale Law and clerked for SCOTUS. He's a smart guy, and he has collected about every brass ring the profession has to offer. He may make some outlandish arguments, but he's a defense attorney who defends people who are guilty as all hell. He's going to have to make those arguments; it's his job. I disagree with his take on the impeachment here (as it was explained, I haven't snuck through the pay wall yet to actually read it), but I think its a fair argument to make; clearly, impeachment is tied to the federal office. Dershowitz may be a clown or political hack or what have you, and I guess the seriousness of my legal mind is debatable, but even if I think he's being a zealous advocate (or full of shit), I'm going to consider what he has to say because he has some serious credentials and some serious experience.
Wink, I'm hardly an expert, but I did OK in Con Law and like to think I can hold my own for a new lawyer. I have no doubt Dershowitz would take me out to the wood shed in a courtroom, but I think he's wrong on this one. I think the Senate can try him here without issue for a few reasons, and I'm curious to hear what others think.
1. I just don't read the Constitution to say the person still has to be in office to be impeached or tried in a case of impeachment. Obviously there is a lot of stuff the Constitution doesn't spell out that gets attributed to it, so I guess it depends how much of a textualist is doing the reading, but I try and stick to the words;
2. The President was impeached while he was still in office, so even if someone has to be in the office to be impeached, he was. I think it's fair to argue that even assuming the framers intended for only those currently in office to be subject to impeachment, there is no reason to think the trial couldn't occur later;
3. I think the limits on the judgment from a conviction the Senate in the Constitution are telling. Because "disqualification to hold and enjoy any office of honor, trust or profit under the United States" it is reasonable to conclude more than the current office is at stake and therefore the process should remain available. It also ensures, imo, that the case isn't moot just because the impeached President is no longer in office. I would argue the framers thought it was important for the Senate to be able to bench people it found worthy of conviction and the importance of that wouldn't stop just because the guilty person had already left the office;
4. Practically speaking, who is going to stop the Senate from trying it? The House is on board and enough of the Senate will be. I find it hard to believe President Biden will send in the DOJ to fight for President Trump, so you'd be left with a minority of Congress and an ex-President trying to take it to SCOTUS. SCOTUS has already invoked the political question doctrine when considering whether the Senate's impeachment procedures were proper in a case, see Nixon v. US, 506 U.S. 224 (surprisingly, not THAT Nixon - that's a different case), and I doubt they're going to get involved on this one since the Constitution still effectively says the courts have no business there (other than the Chief presiding for POTUS).
If I run across any actual experts' takes, I'll try and come back and post them.
Other stuff I saw in here I had some thoughts on that are neither here nor there:
Someone mentioned Biden pardoning Trump. He can't pardon him in a case of impeachment. U.S. Const., Art. 2, Sec. 2. Not sure if the comment was referring to impeachment or not. He could pardon him for other federal crimes.
I think Griz mentioned a bill of attainder issue. I don't think that's an issue here. A bill of attainder is when the legislature usurps the role of the judiciary by "trying" someone by passing a law saying the person is guilty and sentencing them. Impeachment is separate, and the punishment is limited to the effects on the ability to hold the office and its trappings. If they went for something beyond that, then the issue might come up. Plus the things President Trump already allegedly did were already illegal when he did them.