Arizona's tight Senate race likely will not be called until later this week, since there are over 1 million outstanding early voting ballots that still must be counted, CBS News' Fin Gomez reports. As of early Wednesday, McSally had a lead of about 13,000 votes with 63 percent of the precincts reporting.
No matter who wins, Arizona will have its first female senator in history.
The candidates courted swing voters as they vied to replace Republican senator and frequent Trump critic Jeff Flake. Flake has said his brand of Republicanism simply isn't viable anymore in today's polarized political climate.
By October, Sinema had raised roughly $18.9 million, edging out McSally's $16.1 million.
But the milestone of electing the state's first female senator isn't that important to all voters. A recent CBS News poll showed more than half of Democrats are excited about the state electing its first female senator, but most Republicans say it doesn't matter.
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Live results and news from the Arizona Senate race below:
Arizona Senate candidates
Martha McSally (Republican)
McSally, 52, currently represents Arizona's 2nd congressional district, serving on the House Armed Services and House Homeland Security Committees. Before joining Congress, she served for 26 years in the U.S. Air Force, and retired in 2010 as a colonel. She is the first fighter pilot in combat, and the first to command a fighter squadron in combat in U.S. history. She received her master's degree from Harvard University, and attended the U.S. Air Force Academy before that.
Kyrsten Sinema (Democrat)
Sinema, 46, has represented Arizona's 9th congressional district since 2013. A Tucson native, she attended Brigham Young University, then Arizona State University. She also served in the state legislature in Arizona. Sinema was the first openly bisexual member of Congress. Sinema now serves on the House Committee on Financial Services.
Voter confusion in Maricopa County
A polling location in Maricopa County, Arizona, turned away dozens of voters Tuesday morning because they weren't in the right precinct.
But the location, inside the Tempe Historical Museum, was clearly advertised as an open "vote anywhere" site, CBS News correspondent Paula Reid reports. People were lined up before 5 a.m., and at least a dozen voters told CBS News they were turned away and didn't have time to go to another location before work.
Maricopa County is home to roughly two-thirds of Arizona voters, and has been fraught with voting issues over the last few cycles.
What to watch for
A Democrat hasn't won a Senate seat in Arizona for three decades. A Democratic win would be a referendum on President Trump's approach and policies.
If Sinema wins, it could also be in part because of her intense focus on health care.
Keep an eye out, too, for Green Party candidate Angela Green -- yes, her name is the same as the party's -- as a possible spoiler in the race. In an NBC News/Marist poll last month, 6 percent of likely voters put their support behind Green. Although third-party candidates tend to get less support on election day than in polls, Green could make a difference in a close race.
Where to find election results
When the polls close, the latest election results will be posted at the CBS News election center.