Stephanie Bice, a Republican state senator, on Wednesday defeated Representative Kendra Horn of Oklahoma, toppling one of the most vulnerable Democrats in a deeply conservative state and giving Republicans a pickup as they pressed to add to their numbers in the House.
The victory of Ms. Bice, 46, returned the Fifth District to Republicans, who had held it for more than 40 years before Ms. Horn’s stunning upset in 2018, which made her the lone Democrat representing the state in Congress. Republicans had singled out Ms. Horn’s seat, which represents Oklahoma City, as one of their likeliest opportunities to claw back seats and counter any Democratic gains.
Seen as a rising star in the state legislature, Ms. Bice will bolster the minuscule ranks of Republican women in the House, whose numbers have shrunk even as Democrats have made history with the racial and gender diversity of their members. She overcame a tough primary and runoff race by framing herself as more moderate than her opponents, though she later highlighted her conservative credentials and an endorsement from the National Rifle Association.
Ms. Bice accused Ms. Horn of having failed to uphold her promise to be an independent voice in Washington, branding the congresswoman a stalwart ally of Speaker Nancy Pelosi of California and urging voters to end what she called the “Pelosi Majority” in Washington.
“In Congress, I’ll work for you — not Nancy Pelosi,” Ms. Bice declared in a recent campaign ad.
While she was careful to secure the support of Mr. Trump and other top national Republicans, she sought to root her campaign in local issues, emphasizing her support for the state’s oil and gas industry and the work she had done in the Legislature. National Republicans also poured millions of dollars into the race, in an effort to offset Ms. Horn’s fund-raising prowess.
Ms. Horn, for her part, had sought to replicate the playbook that helped her secure the seat in 2018, highlighting her support for the Affordable Care Act and education in the state. She had also upheld her moderate credentials and her willingness to break with her party, including rejecting Democratic stimulus proposals for being too costly and partisan and chiding former Vice President Joseph R. Biden Jr., the presidential nominee, for his calls to “transition” from oil and gas.
But having won the district by a razor-thin two points in 2018, she had a steep climb to re-election. Singled out by Republicans as a critical opportunity to flip a seat in a district that Mr. Trump won by nearly 14 points in 2016, Ms. Horn was unable to hang on.