Last month, Jessica Cisneros, a 26-year-old native of South Texas and immigration lawyer, announced her primary challenge against Rep. Henry Cuellar, a Democrat who represents Texas’s 28th District.

“This is the most important election in our lifetime and I’m proud there are so many candidates running for Congress who understand that real change comes from the bottom on up, not the top on down,” ...

Rep. Henry Cuellar talked to voters in Rio Bravo, Texas, last week.Sergio Flores for The Wall Street Journal In a closely watched Texas Democratic House runoff, Rep. Henry Cuellar leads Jessica ...