Arizona’s knock-down, all-out Senate race is heading into overtime, as a neck-and-neck contest between two congresswomen collides with Arizona’s sometimes glacial vote-counting procedures.

Republican Martha McSally and Democrat Kyrsten Sinema were separated by a small fraction of the votes tabulated as of early Wednesday, with hundreds of thousands of uncounted ballots still outstanding.

Though the vast majority of 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 in the county, where about 60 percent of Arizona’s voters live. Arizona counties with far fewer voters may also face long delays processing those ballots.

That leaves the contentious Senate race a cliffhanger in what’s otherwise shaping up to be another banner Arizona year for Republicans. The GOP has won every statewide race in Arizona over the past decade, and Democrats were hoping Sinema could break that streak.

Democrat Ann Kirkpatrick was elected to the Tucson-area swing district seat vacated by McSally. Democratic Rep. Tom O’Halleran’s mainly northern Arizona seat was too close to call early Wednesday, and the outcome will determine which party gets the majority of the state’s nine member U.S. House delegation.

The election featured heavy statewide turnout of about 60 percent, more in line with a presidential election than a midterm.

The Senate contest was the marquee race, a contest between two champion fundraisers who are no strangers to tight races. McSally lost her first general election by less than 200 votes and won her second by about that many, and Sinema also represents a competitive swing district.

The two are battling over the seat vacated by Sen. Jeff Flake, a Republican who decided not to run for re-election because he realized his criticism of President Donald Trump made it impossible for him to survive politically.

McSally and Sinema have both remade themselves politically. McSally, 52, is a onetime Trump critic who has embraced the president since his election. She has tried to rally Republican voters by emphasizing her military background as the first U.S. female combat pilot while touting her support for the president’s tax cut and other parts of his agenda.

Sinema, 42, is a former Green Party activist who became a Democratic centrist with her first election to the House of Representatives in 2012.

She’s one of the congressional Democrats most likely to vote to back Trump’s agenda but has spent the race hammering McSally for casting a vote for the health bill backed by the president. The repeal of the Affordable Care Act, which didn’t become law, would have weakened protections for people with pre-existing conditions.

In response, McSally criticized Sinema over her shifting views, contending she was still a closet liberal who disrespected the military. Republican ads publicized a 2010 video of Sinema repeating a comedian’s description of Arizona as “the meth lab of democracy.”

McSally also accused Sinema of treason for an offhand comment in a 2002 radio interview with an anti-war talk show host who suggested hypothetically he might join the Taliban. Sinema had responded it would not bother her if she did so.

During her 2016 campaign to be re-elected to her Tucson area swing district House seat, McSally criticized Trump for attacking the parents of an Army captain killed in Iraq and for a videotape in which the future president bragged about sexually assaulting women.

That earlier criticism of Trump hobbled McSally during this year’s three-way Republican primary for Senate, when challengers attacked her not supportive enough of the president.

Sinema faced no real opposition in the Democratic primary and had months to define herself as a nonpartisan, problem-solving centrist on the airwaves while her allies slammed McSally with attack ads over the Republican’s health care vote.

The candidates and their allies spent more than $90 million in a race that could determine which party controls the U.S. Senate. Also at stake is Arizona’s role in national elections. Democrats have repeatedly hoped the state’s growing Latino population and influx of more educated professionals would make it competitive.

The Senate race will test that theory and may help determine whether Democrats target Arizona in the 2020 presidential election.

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For AP’s complete coverage of the U.S. midterm elections: http://apne.ws/APPolitics