Mary Jo Pitzl, The Republic | azcentral.com Published 9:11 p.m. MT July 19, 2017 | Updated 10:26 p.m. MT July 19, 2017
If Sen. John McCain were to leave the Senate before his term ends in 2022, the governor would decide who fills the vacancy.
McCain, who revealed Wednesday that he had been diagnosed with a brain tumor, made no indication he intends to step away.
"Further consultations with Senator McCain's Mayo Clinic care team will indicate when he will return to the United States Senate,” his office said in a statement.
But were circumstances to require that McCain be replaced, that person would have to be a Republican, as McCain is, and would serve until the next general election, which happens every two years in Arizona. Whoever was elected would then fill out the rest of McCain's term, according to state law.
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http://www.azcentral.com/story/news/politics/arizona/2017/07/20/what-if-senator-mccain-leaves-office-how-process-works/494621001/