Image copyright Facebook/ Sheriff Troy E Nehls

A vehicle decal directing an expletive at US President Donald Trump and those who voted for him has triggered a free speech debate in Texas.

The pick-up truck sticker prompted a Facebook post by a local sheriff who asked to speak with the vehicle owner.

In his post, the sheriff threatened to prosecute the owner for disorderly conduct unless she changes the sign.

But free speech advocates argue the anti-Trump driver, 46-year-old Karen Fonseca, within her rights.

The controversy began in the Houston suburbs when Fort Bend County Sheriff Troy Nehls posted a Facebook photo of the truck displaying the sticker.

"I have received numerous calls regarding the offensive display on this truck," wrote the Republican sheriff, who local media outlets report is considering a run for US Congress.

"If you know who owns this truck or it is yours, I would like to discuss it with you.

"Our Prosecutor has informed us she would accept Disorderly Conduct charges regarding it, but I feel we could come to an agreement regarding a modification to it."

The vehicle owner, Mrs Fonseca, told local media she has no plans to speak to the sheriff, telling the Houston Chronicle: "It's just our freedom of speech and we're exercising it."

Mrs Fonseca, who shares the truck with her husband, said she affixed the custom-made sticker about 11 months ago after President Trump was sworn into office.

"It's not to cause hate or animosity," she told the newspaper, adding that she used to work under the sheriff at the county jail.

"It makes people happy. They smile. They stop you," said Mrs Fonseca.

"They want to shake your hand," she told local station KTRK.

She added that people tell her they "wish they had the guts" to put a similar sign on their vehicle.

After the Facebook post drew attention, the sheriff held a press conference where he appeared to back down from his original suggestion that charges were warranted.

"We have not threatened anybody with arrest. We have not written any citations," Sheriff Nehls said.

"But I think now it would be a good time to have meaningful dialogue with that person and express the concerns out there regarding the language on the truck."

Fort Bend County narrowly voted for Hillary Clinton in the 2016 election by 51%, with 45% for Mr Trump, according to the Washington Post.

Texas state law describes disorderly conduct as intentionally using "abusive, indecent, profane, or vulgar language in a public place, and the language by its very utterance tends to incite an immediate breach of peace".

The law also prohibits making "an offensive gesture or display in a public place" that could have the same effect.

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