As the San Antonio Express News reports, the billboard in question is apparently in reference to a New York Post article published earlier this year that claimed Cisneros had an affair with a 40-year-old teacher when she was 18 years old.
In addition to the billboard, residents in the district have also been receiving text messages accusing Cisneros of having "no integrity knowingly sleeping with a married man."
Cisneros slammed Cuellar and his allies for trying to "bully" her out of the race by making salacious accusations.
“My opponent and his supporters are so desperate to hold on to power that they have resorted to bullying me in hopes that they’ll be able to win,” she said. “I’m here to say that I will not be intimidated.”
Cuellar, however, quickly distanced himself from the billboard, which he said he does "not condone."
“I’m running an issues based campaign focused on delivering for the people of South Texas,” he said.
The primary challenge to Cuellar has drawn heightened scrutiny in the wake of the leaked draft Supreme Court opinion overturning Roe v. Wade.
Democratic leaders have stood by the incumbent lawmaker even though he is a rare Democrat who has taken stances against abortion rights.