The Republican Senate primary in Louisiana has narrowed to two candidates after the incumbent, Bill Cassidy, who voted to convict Donald J. Trump in his impeachment trial five years ago, was defeated on Saturday.

The two top vote-getters, Representative Julia Letlow and State Treasurer John C. Fleming, are headed to a runoff scheduled for June 27. The winner will be heavily favored in the general election because of the state’s strong conservative tilt.

Neither candidate secured the majority of the vote needed to advance to the November general election under Louisiana’s system. Ms. Letlow won about 45 percent of the vote, and Mr. Fleming finished with around 28 percent. (Senator Cassidy picked up roughly 25 percent.)

Here’s what to know about Ms. Letlow, 45, and Mr. Fleming, 74.

Mr. Trump endorsed Ms. Letlow.

The congresswoman, a former teacher and university administrator from Monroe, was backed by the president even before she formally entered the race. In January, Mr. Trump urged Ms. Letlow to challenge Mr. Cassidy, writing on social media: “RUN JULIA RUN!!!”

After she finished first on Saturday, the president celebrated not only Mr. Cassidy’s loss but Ms. Letlow’s step toward the Senate.

“Julia Letlow is a spectacular person, and will never let you down!” Mr. Trump wrote on social media.

She first ran for Congress after her husband, a representative-elect, died of Covid-19 complications.

Ms. Letlow, the first Republican woman elected to Congress from Louisiana, first won her seat in early 2021. She emerged from a special election to claim the seat her husband, Luke Letlow, won the previous November; he died the following month of complications from Covid, days before he was to take office.

Ms. Letlow represents a deep-red, L-shaped district in northern and eastern Louisiana. On Saturday, she performed best in the rural northern Louisiana areas that make up much of her district, winning more than two-thirds of the vote in some counties.

She once supported diversity, equity and inclusion programs.

While interviewing to be president of the University of Louisiana at Monroe in 2020, Ms. Letlow spoke favorably about D.E.I. efforts, expressed concerns about “unconscious bias” on campus and denounced the school faculty’s lack of diversity as “shameful.”

“We don’t have enough women of color at the top,” she said.

Ms. Letlow did not receive the job, and her comments became an attack line for her opponents in this year’s primary. She told Louisiana First News that though she initially supported D.E.I. programs, she quickly decided they had been “hijacked by the radical left and turned into indoctrination.”

Mr. Fleming was a founder of the ultraconservative House Freedom Caucus.

A physician before entering politics, Mr. Fleming ran for Congress in 2008 and won, replacing Representative Jim McCrery, a Republican.

Mr. Fleming, who lives outside Shreveport, went on to serve four terms, representing the hilly northwestern corner of the state.

He joined congressmen including Jim Jordan, Ron DeSantis and Mark Meadows in forming the Freedom Caucus, a group of hard-line Republicans banded together to push for changes in how the House Republican leadership operated.

He served in the first Trump administration.

Mr. Fleming served as White House deputy chief of staff in the final months of Mr. Trump’s first term. He was in the role in the weeks after the Jan. 6, 2021, attack on the U.S. Capitol.

He also worked in the Health and Human Services during Mr. Trump’s first term and, for a period, as assistant secretary of commerce for economic development. As he campaigns for Senate, Mr. Fleming has leaned on those ties to Mr. Trump.

“When others cut and ran, I never wavered,” Mr. Fleming’s campaign website says. “I stayed, working in the White House until the last day.”

He has pumped his own money into his campaign.

Mr. Fleming, who owns 30 Subway restaurants, has lent his campaign millions of dollars, according to federal campaign finance records.

“The people of Louisiana deserve a senator who cannot be bought, will not be bossed, and will never back down,” Mr. Fleming said in a statement Sunday.