Republicans flip a Democratic-held House seat in South Texas, at least for now.

A U.S. House district in South Texas will send a Republican to Congress for the first time in its 10-year history.

Mayra Flores, a Republican and respiratory-care health aide, scored a significant victory in a special election on Tuesday for the party, which has been trying to capitalize on its successes in 2020 in the Democratic stronghold of the Rio Grande Valley. She will be the first Latina Republican from Texas in Congress.

Ms. Flores defeated three opponents in the special election to replace former Representative Filemon Vela, a Democrat who retired this year before the end of his term. She captured more than 50 percent of the vote in Texas’ 34th Congressional District, according to The Associated Press, and will avoid an expected runoff with Dan Sanchez, a Democrat and former commissioner in Cameron County.

Her win may only be temporary, however.

The special election was held to determine who would fill the remainder of Mr. Vela’s term until the end of this year. Voters in the general election in November will decide who will become the district’s permanent representative beginning in January. Representative Vicente Gonzalez, who currently represents a neighboring district, is the Democratic nominee for November, and is widely favored to win the race against Ms. Flores, who is also running to fill the seat permanently in November.

Republicans have directed enormous sums of money and attention to the race in recent weeks, seeking an early victory in a district that includes the border city of Brownsville. Ms. Flores raised 16 times the amount of money that Mr. Sanchez did. And she and her allies have spent more than $1 million on television advertisements, while Democrats have largely stayed off the air.

Republicans believe they have found an ideal candidate for the region in Ms. Flores, who immigrated to the United States from Mexico as a young child. Her parents spent years working as migrant farmworkers in Texas. She is the wife of a Border Patrol agent and has campaigned on strict immigration enforcement in the overwhelmingly Mexican American district.

“We’ve been voting California values, Austin values, but not South Texas values,” she said at a campaign event earlier this year. “This is our country.”

Like other Hispanic Republicans in the region, Ms. Flores has continually attacked Democrats for not paying enough attention to South Texas, where former President Donald J. Trump made significant inroads with Hispanic voters in 2020. And she has fiercely embraced the Trump wing of the party and its false stolen-election claims.

The special election presented significant challenges for Democrats.

Because of new lines that were drawn in the redistricting process, Democrats hold a slimmer margin in the special election than they will in November. Mr. Vela surprised Democrats when he announced his decision in March to become a lobbyist before his term finished. Mr. Gonzalez decided against resigning from his current seat to run in the special election.

Democrats were largely reluctant to put resources into a special election in a district that they view as a safe bet in November. Instead, they have been more focused on the neighboring 15th District, Mr. Gonzalez’s current seat. That district is seen as one of the few competitive ones in Texas this year.