John McCain To Resign Senate Seat on the 4th of July, Says Aide. Cindy McCain Offered his Seat - Santa Monica Observer
And down the article it says this, so it seems Vox may be the actual source, what do you think?
“If McCain leaves office before May 30, the Post indicated, his Senate seat will be on the ballot in Arizona in November 2018, as will that of his retiring junior colleague, Republican Sen. Jeff Flake. After May 30, the new senator — who would be appointed by Republican Gov. Doug Ducey — would serve through 2020.” https://www.vox.com/policy-and-politics/2018/4/4/17197218/john-mccain-brain-cancer-arizona-senate-vacancy