Vandals have painted the word “traitor” across the front of a pro-EU MP’s constituency offices.
Labour MP Chris Bryant, who wants the UK to remain in the EU, said the graffiti on his Tonypandy offices would not change his mind on Brexit.
In a defiant tweet on Saturday morning, the Rhondda MP said the people who painted the abuse “underestimate” him.
South Wales Police said it was investigating and asked anyone with information to come forward.
Mr Bryant blamed “anyone who has poured a bit more bile into the pot” for contributing to a lack of respect.
None of his staff were in the building at the time, but he said it was unpleasant for his staff who had already faced abuse since the referendum in 2016.
He had already increased security at his Dunraven Street office, he added.
“I don’t know why anybody would think spending 10 minutes spray-painting names on a shutter will change my mind,” he said.
“We didn’t used to be a democracy like this – we used to respect each other’s opinions.”
He told BBC Wales he expected the hostile political atmosphere to continue until Parliament comes to a decision on Brexit.
MPs and campaigners have offered their support on social media.
Former Labour MP Chukka Ummuna tweeted: “This vile intimidation and abuse is appalling. Good for you @RhonddaBryant for standing up to it and for defending our democracy”.