Actually, if witnesses are allowed, the ONLY ones who can be allowed are those that have already testified to the House. The Constitution gives the power of impeachment solely to the House. The evidence put forth by the House is then presented to the Senate for final judgement. If new evidence comes along, it is STILL the job of the House to gather and determine if it should be put forth before the Senate in the form of an Article of Impeachment. Constitutionally, the Senate is banned from the new evidence gathering role. This means that if the House wants to call Bolton as a witness, they can ONLY do so as a function of the HOUSE. If they call him to testify and then determine that it warrants an Article of Impeachment, then they must go through the process given to them by the Constitution. They cannot pass this function along to the Senate.
This means that "new" witnesses are unconstitutional if allowed by the Senate. They can't do it.
Interestin. First I've heard this. How are they even contemplating new witnesses??
The second point is demonstrably false. The Senate does all sorts of unconstitutional things on a regular, almost daily basis. It's the new normal.
There is no remedy to prevent the senate from sending subpooeanas to new witnesses. Naturally, a new witness can sue to quash, but courts aren't a bastion of constitutionality either.