With Rep. Ciro Rodriguez filing a lawsuit to overturn the result in the CD28 Democratic primary, the case would appear to turn on the question of what a valid voter registration is.
The issue of residency is a murky one, some say, involving family ties and deep-rooted cultural traditions.
At Zacatecas and New York streets on the city's Southeast Side, one home boasts a bright blue plastic tarp that serves as a front door.
A steady stream of cars chugged through the neighborhood as Rosaura Castaņon emerged from behind the tarp to answer a reporter's questions.
"That's my brother, that's my brother-in-law, that's my sister-in-law," Castaņon said, rattling off family members as she peered at a list of people with five different last names. "We all voted."
She's one of 12 family members who share the modest home.
Rodriguez attorney Buck Wood acknowledges family ties are at the heart of many of his examples of alleged illegal voting, such as a son or daughter who moves from his or her parents' home but maintains voter registration there.
"The tradition of people maintaining an address other than where they live seems to be very prevalent here " in Laredo, he said. "It's very family-oriented and people maintain their connections."
While it may not be fraud, it's certainly not legal, he said.
"If you go to a poll and you claim you still live at an address, that's illegal," Wood said. "That vote's going to be thrown out."
Cuellar attorney Steve Bickerstaff disputes that.
"It's very, very common for the registration to remain at the residence of the parents," he said. "Any ruling that would say that's a violation of the law would have implications statewide for minority voting."
Trying to determine if family members have moved out of a home but remain registered there could lead to all sorts of problems, Bexar County Elections Administrator Cliff Borofsky said.
"I think it's probably a fairly common phenomenon," he said. "Statewide, I'm sure there are many cases where families vote together."
Borofsky said officials try to keep tabs on residency by sending out voter registration cards to last-known addresses. If the person no longer lives there, the current residents are supposed to return the card to the county.
"It would be a mess, and in order to correct it, the (Texas) secretary of state or a court would have to ask every elections administrator to investigate whether people really live where they say they do."
State law tends to protect rather than inhibit the right to vote. To be eligible to vote, states the Election Code, a person must "be a resident of the territory covered by the election," in this case, Congressional District 28. It doesn't refer to a specific address or time frame.