Text messages leaked earlier this month showed that Cunningham, a married man, had an extramarital relationship with a consultant.
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Republicans hold a 53-47 majority in the Senate, and the contest between Cunningham and Republican Sen. Thom Tillis has been among the most-watched in the country.
An Army Reserve lieutenant colonel who projected what seemed to be a wholesome appeal, Cunningham was widely viewed as the kind of recruit Democrats needed to make inroads in conservative-leaning Southern states such as North Carolina.
Yet the text messages and interviews offered a glimpse that is at odds with the image of a devoted family man. A conservative website, NationalFile.com, published text messages between Cunningham and Arlene Guzman Todd, a married public relations strategist from California, that suggested a personal relationship.
"I have hurt my family, disappointed my friends, and am deeply sorry. The first step in repairing those relationships is taking complete responsibility, which I do. I ask that my family's privacy be respected in this personal matter," Cunningham, who has two teenage children and has been married for two decades, said.
What was unclear was whether the revelation of an affair between two consenting adults is something North Carolina voters would care about, particularly after the state swung in 2016 for President Donald Trump, who has been married three times and allegedly paid hush money to a porn actress to keep silent about an extramarital tryst.
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The latest forecast from FiveThirtyEight is evidence the affair might not change the minds of voters.
It shows that Cunningham has about a two in three chance of victory. The forecast has remained stable since September, even after word of the affair went public.
The FiveThirtyEight model produces probabilistic forecasts, as opposed to hard-and-fast predictions about who will win or lose. In the same sense that a weather forecaster might tell you there's a 30 percent chance of rain tomorrow, the FiveThirtyEight model estimates the chances of each candidate winning. It will update continuously up until Election Day.
FiveThirtyEight, a data-driven news site founded by Nate Silver in 2008, joined ABC News in 2018.