Gallego spokeswoman Rebecca Acuna says Clinton will hold a rally for the state lawmaker Thursday at South San Antonio High School. Gallego is trying to unseat Republican U.S. Rep. Francisco Canseco in the sprawling 23rd Congressional District.
The race is among only a handful of toss-ups in Texas as the Nov. 6 election approaches.
Early voting begins Monday. Acuna says the Clinton event is free but supporters must obtain tickets at select early voting sites.
It's the third time since 2006 that Clinton has rallied Democrats in what has become a perennial swing district. His last visit in 2010 couldn't stop a Republican victory.