"I think this Republican race here is the epitome of the infighting within the Republican Party," said Arizona State University political scientist Rodolfo Espino.
But like the Republican Senate primary in Nebraska earlier this year, the lines between tea party and establishment have been blurred. While Smith has lined up substantial support from the business community, Ducey, has the backing of the president of of the Arizona Chamber of Commerce, a leading business group. Business groups have been at odds with the tea party in other key races this year. Meanwhile, Jones, a former executive at GoDaddy, has been courting conservative voters.
Nevertheless, a Ducey win -- limited available polling shows he is the frontrunner -- would be held up by tea party leaders as a victory in an otherwise disappointing election cycle so far, in which the GOP establishment candidate won most high-profile congressional races. It would also likely give them a powerful ally next year. The winner of the Republican race will be heavily favored to defeat Democrat Fred DuVal in Arizona's first open governor's race in 12 years.
Arizona is one of four states where voters are heading to the polls with primary season winding down. In Florida, former governor Charlie Crist (D) and Gov. Rick Scott (R) are expected to advance to what's expected to be one of the most expensive contests in history. Voters in Oklahoma and Vermont will settle lower-profile primaries.
After unseating Republicans senators in primaries during the last two election cycles, the tea party failed to do so this year. Tea party candidates have had a bit more success in House primaries, but even there, wins have been hard to come by.
While Democrats have murmured about turning Arizona purple, it remains ruby red territory, making it a plum location for the tea party to stake its claim.
The candidates have been relying on their high-profile supporters to put them over the top. Cruz did a tele-town hall for Ducey and Brewer has been campaigning hard for Smith down the stretch.
Immigration has become a focal point in the Arizona campaign, with Ducey and Jones adopting harder line positions on border security and illegal immigrants than Smith. Ducey is backed by Maricopa County Sheriff Joe Arpaio, known nationally as an immigration hardliner.
Smith's strategy has been to win the support of moderate Republicans while the rest of the field -- there are five other candidates, including Ducey and Jones -- splits up votes on the far right.
Early voting is a very popular option in Arizona, which observers say bodes well for Ducey, who has appeared to to be the leader for much of the last month.
Polls close at 7 p.m. local time in Arizona. Stay tuned to Post Politics for election results this evening.