AUSTIN — Democratic challenger Jessica Cisneros, a 28-year-old immigration attorney running for Congress with heavy support from progressives, has forced a primary runoff against U.S. Rep. Henry Cuellar in South Texas.

Neither candidate Tuesday night got more than 50% of the vote needed to win outright.

The race was among the most heated in Texas’ first-in-the-nation primary. Cuellar is among the most conservative Democrats in Congress, and a Cisneros victory would bring a seismic liberal shift to the district that runs from San Antonio to the Texas border.

Cisneros is a former intern of Cuellar’s who also ran against him in 2020. She narrowly lost, and this time ran a more aggressive campaign that attacked Cuellar over FBI agents searching his home just before voting began.

The Associated Press has learned that the investigation relates to the former Soviet Republic of Azerbaijan. Cuellar has not been charged with a crime and has denied wrongdoing. The runoff is in May.