Katie Britt leveraged her Alabama connections and political savvy to trounce Mo Brooks.

Ms. Britt entered the primary with little name recognition and long odds against Mr. Brooks, who boasted more than a decade of experience in the House and gained Mr. Trump’s backing after he riled up the crowd at the former president’s rally before the attack on the Capitol on Jan. 6, 2021.

But Mr. Trump rescinded his support for Mr. Brooks in March as Mr. Brooks struggled to gain traction under an avalanche of attack ads and criticism of his decision to urge an audience at a Trump rally to leave the 2020 election behind. “Katie Britt, on the other hand, is a fearless America First Warrior,” Mr. Trump said in a statement this month as he endorsed Ms. Britt.

That move did not completely wipe out Mr. Brooks, who still managed to clinch a second-place finish in Alabama’s May 24 primary, garnering 29 percent of the vote. Ms. Britt pulled in 45 percent, short of the majority that would have avoided a runoff between the two top vote-getters.

Ms. Britt fashioned herself as an “Alabama First” candidate, playing off Mr. Trump’s “America First” presidential campaign slogan, and centered her run on her Christian faith, hard-line border enforcement policies and ties to the business community.

As an aide for Mr. Shelby, one of the Senate’s most senior members, she worked on some of his signature issues, including a sweeping Republican package of tax cuts in 2017, confirmation of conservative judges and a push for a border wall along the U.S.-Mexico border.

She most recently served as the head of the Business Council of Alabama, a powerful lobbying group, and led a “Keep Alabama Open” campaign in November 2020 against coronavirus pandemic restrictions that required nonessential businesses to close or limit services. She also opened the council’s resources, typically reserved to paying members, to all small businesses amid the health crisis.