Kendra Horn speaks during a forum for Oklahoma 5th congressional district seat Democratic candidates in Edmond, Okla. Horn is facing incumbent Republican Rep. Steve Russell.

Democrat Kendra Horn is projected to defeat Republican Rep. Steve Russell in Oklahoma's 5th district, according to NBC News. The victory represents a stunning upset that was not predicted by most Democratic election models.

The central Oklahoma district is a Republican stronghold, and the widely respected Cook Political Report rates it R +10, which means that in recent elections, elections in the 5th district were 10 points more Republican than the national average.

Despite the fact that the district was considered safely Republican, Horn has outraised Russell this cycle, as have many Democrats across the country.

In 2016, Russell won the seat by more than 20 points, a reflection of the district's strong Republican demographics.

Horn also likely benefited significantly from a late stage injection of cash from billionaire Michael Bloomberg's Independence USA political action committee, which spent $400,000 on ads promoting Horn in the final weeks of the race.

This is a developing story. Please check back for updates.