The 2018 United States Senate election in Arizona took place on November 6, 2018.^[2] Incumbent Republican Senator Jeff Flake did not seek reelection to a second term. The election was held concurrently with a gubernatorial election, other elections to the U.S. Senate, elections to the U.S. House of Representatives, and various other state and local elections.

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2018 United States Senate election in Arizona

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Turnout	64.85% [1]
Nominee Kyrsten Sinema Martha McSally Party Democratic Republican Popular vote 1,191,100 1,135,200 Percentage 49.96% 47.61%
County results Congressional district results State Senate district results Precinct results Sinema:      40–50%      50–60%      60–70%      70–80%      80–90%      >90% McSally:      40–50%      50–60%      60–70%      70–80%      80–90%      >90%      No data
U.S. senator before election Jeff Flake Republican Elected U.S. Senator Kyrsten Sinema Democratic

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Primaries were held on August 28, 2018,^[3] three days after the death of longtime U.S. Senator John McCain.^[4] Martha McSally won the Republican nomination, while Kyrsten Sinema won the Democratic nomination. Green Party candidate Angela Green was also on the ballot, but ended her campaign and endorsed Sinema before Election Day.^[5]

The Associated Press called the race for Sinema on November 12, 2018,^[6] and McSally conceded that day.^[7] Sinema became the first Democrat to win a U.S. Senate seat in Arizona since 1988. McSally was subsequently appointed by Governor Doug Ducey to the other vacant Senate seat in Arizona, left open after McCain's death and then held on an interim basis by Jon Kyl.

Arizona, located along the United States border with Mexico, has a unique political history. Upon its admission to the Union in 1912, the state was dominated by Democrats who had migrated there from the South, and aside from the landslide victories of Republicans Warren G. Harding, Calvin Coolidge, and Herbert Hoover, the state voted for Democrats until 1952, when Dwight Eisenhower carried it, and began a lengthy streak of Republican victories interrupted only by Bill Clinton's narrow victory in 1996. Since then, the state had remained Republican, and was won by Donald Trump with a 3.5% margin in 2016, although Trump's margin of victory was much smaller than that of past Republican presidential nominees.^{[8][better source needed]}

Incumbent Republican Senator Jeff Flake announced in October 2017 that he would retire at the end of his current term instead of seeking reelection for another term in 2018.^[9] Flake had previously indicated his intent to run for reelection in March 2017. However, he was considered vulnerable due to persistently low approval ratings, a poor relationship with President Trump, and the threat of a primary challenge from former state senator Kelli Ward, who promised to run on a more pro-Trump platform. Additionally, he had won his first term in 2012 by only 3 percentage points, even though Republican presidential nominee Mitt Romney won Arizona by 9.^[10][11][12]

On December 18, 2018, Governor Doug Ducey appointed McSally to fill Arizona's other Senate seat. The seat was left vacant after the resignation of Jon Kyl, who himself had been appointed following the August 25, 2018 death of John McCain.^[213] Both Sinema and McSally were sworn in with the 116th United States Congress on January 3, 2019,^[214] marking the first time in history that Arizona was represented by two women in the United States Senate and making Arizona the second state to be represented by two women from different parties.^{[citation needed]} Sinema and McSally are only the second pair of senators from the same state in history to serve together after running against each other the previous year;^{[citation needed]} the first such instance occurred in Oregon in 1996–1997.^[215] Ducey stipulated that Sinema would be sworn in first, making her the senior senator; this way, he said, the decision of Arizona's voters would be respected.^{[citation needed]}

Under Arizona law, McSally's appointment was only valid for the duration of the 116th Congress^{[citation needed]} and a special election for her seat was held in November 2020 to determine who would finish the remainder of McCain's unexpired term (which expired in 2023). McSally was defeated by Democrat Mark Kelly in that special election.^[216][217]

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