Veteran Democratic Sen. Judith Zaffirini is known by her friends, loved ones and constituents simply as "Z."
But authorities told her sister recently that the symbol, which translates in Spanish to "zeta," could cause the truck to be mistaken for vehicles belonging to the deadly "Zeta" cartel, based on the Texas-Mexico border.
Zaffirini told Reuters on Saturday she was grateful for the warning, but found it "disappointing" and "mind-boggling."
"Sadly, we will remove the 'Z' from our campaign vehicle and will not use it at rallies or other public places," she said.
Late last month, police in downtown Laredo pulled over Zaffirini's sister, Josie Pappas. The officer pointed to three bumper stickers on the pick-up's back window.
The stickers, bearing the word "Zaffirini," were applied in a big "Z" formation on the glass of the blue Ford F-150 truck.
Pappas told Reuters the officer asked her if she had noticed "cars full of men passing me by and staring back" at her car recently.
The bewildered Pappas answered, "No."
The officer told her that in Mexico, the Zetas had taken to wearing the letter "Z" on their clothing and their cars. He advised her it was best to remove the stickers "at this time."
He hadn't been the first to mention it, and she figured it was a sign that it was time to take them down.
"I'm having a little trouble removing it," Pappas said Saturday, adding that she would just have to "hold my nose and do it."
"We just don't have any choice. Those people have changed our lives completely," she said.
Laredo is directly across the Rio Grande river from Nuevo Laredo, a city of more than half a million people and a birthplace of the Zetas, formed in the late 1990s with members of the Mexican military.
The drug war in Mexico has killed more than 37,000 people in the past five years.
The sticker-symbol has adorned all of Zaffirini's campaign vehicles since her days as a money-strapped freshman campaigner in 1986, the senator said.
"Because we only had 12-cent bumper stickers left, my sister, Josie Pappas, thought of improvising for signs by making a "Z" out of three bumper stickers, repeating 'Zaffirini' three times," said Zaffirini, who runs a communications firm. "The resulting 'Z' was attractive and communicated our message."
Zaffirini is known in the Texas Senate for her refusal to back down from a fight and a calm demeanor in the face of hot heads in the Legislature.
"The initial is especially meaningful to me because my favorite fictional hero is "Zorro," who splendidly carved a "Z" to mark the scenes of his good works," she said.
"Who would have thought that an initial that characterizes such a literary hero could be interpreted so negatively in a potentially dangerous context?"
In an email she added: "I will, however, continue to use it personally."
She signed the note, "Z."
(Editing by Jerry Norton)