June 1, 2018
The special election deadline has passed in Arizona, at least as it is interpreted by Republicans. Should Sen. John McCain leave office this year, Gov. Doug Ducey will appoint his replacement. That individual will not need to face the voters until the next regular election for the seat, which is in November, 2020.
According to Politico, "The likely lack of a second open Senate race this year is politically significant: Arizona has become an increasingly competitive battleground state, and another open Senate race this year would increase the chances that Democrats could take back the Senate."