As far as I know, the US Constitution says nothing about recalling Federal elected officials. So since it does not explicitly prohibit recalls, it’s a bit strong to accuse recall proponents of “violating” the Constitution.
Those seeking to recall McCain and Flake cite as their authority the Arizona state constitution, which allows for recalls.
Term limits were ruled unconstitutional. Same thing as recall so it’s not too extreme to say recall is unconstitutional.
See post 27.