The extremely tight race between Republican Martha McSally and Democrat Krysten Sinema is too close to call.
The two were separated early Wednesday by a small fraction of the votes tabulated, with hundreds of thousands of uncounted ballots still outstanding.
Most Arizona voters cast their ballots early by mail. Those who receive early ballots but then drop them off in person at polling stations on or close to Election Day can jam up the system.
That's because the state's most populous county, Maricopa, can take days to count those ballots while they simultaneously tabulate Election Day votes.
The so-called "late earlies" may not be counted until Thursday by the county.