Actually last I checked recall of a US Senator is both Constitutional and legal in AZ. You see those folks in AZ are rascals. They recognized there might be an issue with recalling Federal officeholders. So before they run they have them sign a statement as to whether they are willing to stand for recall election. I don’t know whether mcvain agreed to but virtually all do.
So what is the next argument against a recall? It would be hard to do?
Well, being a rascally Arizonan myself, I’d suggest a US Constitutional Amendment specifically stating the rules for a recall, because pledge or not, a federal office-holder cannot be compelled to leave office as the result of a recall election.
All this ground was covered, and answered, in a federal lawsuit brought by one Tom Foley in WA state when term limits was the subject at hand, another state-level attempt to restrict the holding of a federal office.