Democrat Kendra Horn defeated incumbent Republican Rep. Steve Russell Steven (Steve) Dane RussellGOP women's group launches six-figure campaign for House candidate Bice Bice wins Oklahoma GOP runoff to face Horn in November House Democrats target Midwestern GOP seats MORE in Oklahoma's 5th Congressional District on Tuesday in a major upset.

Horn won 50.7 percent of the vote to Russell's 49.3 percent, flipping the district blue for the first time since the 1970s.

Horn was seen as unlikely to unseat Russell, who was seeking a third term and who had won reelection in 2016 by more than 20 points. Polls leading up to Tuesday's election consistently showed Russell with a double-digit lead.

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The Oklahoman reported last week that Independence USA, the Michael Bloomberg-affiliated super PAC, was spending $400,000 to air a television ad supporting Horn.

Prior to Tuesday, election handicappers at the nonpartisan Cook Political Report rated the race as likely Republican. FiveThirtyEight also had the race as likely Republican entering Tuesday.

According to FiveThirtyEight's forecast, Horn's victory was the biggest upset of the night.