John Mccain’s election as a senator was not legal according to the AZ constitution. AZ limits their federal senators and representatives to 2 terms according to ARTICLE VII. SUFFRAGE AND ELECTIONS, Term limits on ballot appearances in congressional elections. Section 18. (See below).

He was elected in 1998, 2004, 2010, 2016. That’s 4 terms after 1993 (For purposes of this section, terms beginning before January 1, 1993 shall not be considered) (see below).

AZ constitution

ARTICLE VII. SUFFRAGE AND ELECTIONS
Term limits on ballot appearances in congressional elections.
Section 18. The name of any candidate for United States senator from Arizona shall not
appear on the ballot if, by the end of the current term of office, the candidate will have
served (or, but for resignation, would have served) in that office for two consecutive
terms, and the name of a candidate for United States representative from Arizona shall
not appear on the ballot if, by the end of the current term of office, the candidate will
have served (or, but for resignation, would have served) in that office for three
consecutive terms. Terms are considered consecutive unless they are at least one full term
apart. Any person appointed or elected to fill a vacancy in the United States congress who
serves at least one half of a term of office shall be considered to have served a term in
that office for purposes of this section. For purposes of this section, terms beginning
before January 1, 1993 shall not be considered.