Arizona Democratic Rep. Kyrsten Sinema on Thursday night gained a slim lead of just 9,610 votes over Republican Rep. Martha McSally in a too-close-to-call Senate race.

Sinema’s lead translates to less than 0.5 percent. More than 1.8 million votes in the race have already been counted. Although McSally led by 17,703 votes earlier on Thursday, approximately 500,000 votes have yet to be counted in Arizona, both candidate’s campaigns stated.

The majority of the outstanding ballots come from Maricopa County, Arizona’s largest county that includes Phoenix and Sinema’s congressional district.

Sinema, 42, is one of several LGBT candidates who ran in this year’s midterm elections, like Colorado Governor-elect Jared Poliswho is the state’s first openly gay Governor. Should she win her race, she would become the first openly bisexual person elected to the U.S. Senate.

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The race between Sinema and McSally, 52, was reportedly one of the most costly in the nation: each campaign spent around $30 million. The pair are competing to replace retiring GOP Sen. Jeff Flake, who clashed with, but also supported, President Donald Trump on many issues.

Arizona is one of three states with a Senate race that has yet to yield a winner. In Florida, GOP Gov. Rick Scott leads Democratic Sen. Bill Nelson by just two-tenths of a percentage point, although it was announced Thursday that the race will hold a recount. In Mississippi, Republican Sen. Cindy Hyde-Smith and Democratic challenger Mike Epsy are set to face off in a runoff at the end of the month. Hyde-Smith is the favorite to win the runoff.

The GOP has already won 51 Senate seats, while Democrats took 46.