It's not unconstitional. But neither is the proposal here. That's the problem: the rules allow a lot of things that are considered unfair and un-collegial. If that's successful, then eventually the tactics will be considered by the other side.

It's a political judgment call of whether it will be a net benefit to them. (My guess is that they'll conclude that packing the court is not but term limits for future nominations are.) But it will undoubtedly be constitutional -- and pushed as far as they think it can be.