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Hillary Clinton in Arizona.
The latest RealClearPolitics average has Republican nominee Donald Trump over Democratic nominee Hillary Clinton by 4 points. However, earlier last week, the average had Clinton leading by less than 1 point.
The state's 11 electoral votes will prove critical to Trump. Should he lose the state, there is virtually no path for him to reach the needed 270 to win. Clinton can reach 270 more easily than Trump if she were to lose the state.
The only Democrat to win Arizona since 1948 was Clinton's husband, Bill, in 1996.
Polls in the Grand Canyon State close at 10 p.m. EST.
Arizona is also home to a Senate race between longtime Republican Sen. John McCain - the 2008 GOP nominee for president - and Democratic Rep. Ann Kirkpatrick.
The most recent RealClearPolitics polling average has McCain up a comfortable 10 points on Kirkpatrick.