Representative Julia Letlow rode President Trump’s endorsement to victory in a Republican primary runoff election in Louisiana on Saturday, according to The Associated Press, taking a major step toward replacing Senator Bill Cassidy.

Ms. Letlow’s win over State Treasurer John Fleming, a hard-right former congressman, came a month after the two contenders advanced past Mr. Cassidy, a second-term Republican who was targeted by Mr. Trump for defeat.

She enters the general election race as a heavy favorite in deep-red Louisiana, which has not been represented by a Democrat in the Senate in more than a decade. If Ms. Letlow reaches the Senate, she would become the first Republican woman to represent Louisiana in the chamber.

Jamie Davis, a Louisiana farmer who tends to some of the same land his grandfather worked as a sharecropper, won the Democratic runoff, according to The A.P., and will be his party’s nominee.

The outcome of the Republican race is welcome news for Mr. Trump, whose endorsement remains a coveted prize in G.O.P. contests, even after he had some recent losses in primaries for governor in Iowa and Georgia. Seeking to oust Mr. Cassidy after he voted to convict Mr. Trump in his 2021 impeachment trial, the president had urged Ms. Letlow to challenge the sitting senator. In January, he wrote on social media: “RUN, JULIA, RUN!!!”

She led comfortably in the first round of voting on May 16, finishing about 20 points ahead of Mr. Cassidy and about 17 points in front of Mr. Fleming. But she did not secure the majority of the vote needed to avoid a runoff.

More endorsements came after Mr. Cassidy’s defeat. Senators Marsha Blackburn of Tennessee, Joni Ernst of Iowa and Cindy Hyde-Smith of Mississippi all backed Ms. Letlow. Representative Steve Scalise of Louisiana, the No. 2 House Republican, endorsed her, too, calling her a “strong conservative fighter.”

But polls showed a tight race in the runoff’s closing days, as Mr. Fleming, who helped found the hard-right House Freedom Caucus, seemed to connect with some voters by arguing that he was the more conservative option.

He focused on favorable comments Ms. Letlow made about diversity, equity and inclusion while interviewing in 2020 to be president of the University of Louisiana at Monroe. (She had since disavowed D.E.I., saying it had been “hijacked by the radical left and turned into indoctrination.”)

“I have a clearly more conservative, MAGA-aligned voting record than she does,” Mr. Fleming argued in an interview in the runoff’s final days, deriding Ms. Letlow as “woke.”

She responded to his attacks by asserting that she was the true candidate of the MAGA movement, and by casting him as a feckless career politician.

“It’s time for a new generation of Conservative Leadership,” she wrote on social media this month.

By Thursday, Mr. Trump again weighed in on this race, joining a virtual rally to reassert that Ms. Letlow was the MAGA candidate. In the rally, the president described her as a “great person” and an “incredible warrior for Louisiana’s farmers and fishermen and military.”

“I know Julia very well,” Mr. Trump said. “I’ve known her for a long time. And I’ve seen her tested at the highest level.”