At the Pennsylvania state Capitol two gun-rights rally participants held up a banner calling for a Philadelphia lawmaker to be "hung from the tree of liberty."
Black lawmakers denounced the message as racist and "a terroristic threat," recalling lynching. They demanded a police investigation.
The banner said Democratic Rep. Angel Cruz, who is sponsoring legislation to require gun registration and a $10-a-gun annual fee, "should be hung from the tree of liberty for treasonous acts against the constitution."
Rep. Thaddeus Kirkland, chairman of the Legislative Black Caucus, said at a news conference that the banner was "an act of racism, an act of hatred, an act of bigotry and a terroristic threat on the life of a colleague."
"No one has the right to call for the lynching of another human being - no one," he said.
Paul Estus, one of two men who held up the banner, said during the morning rally that the lynching tree was "just a figure of speech."
"You've got to make a stand," he said.
A spokesman for the Capitol Police in Harrisburg, Edward Myslewicz, said the department was unaware of the banner but would follow up if it receives a complaint. Kirkland said a letter would be sent to the state police late Tuesday.
Hundreds of gun-rights advocates packed the Capitol Rotunda to hear a group of state lawmakers, mostly Republicans, advocate proposals to expand those rights.
Cruz has said he introduced his bill to address escalating gun violence in Philadelphia, which, with 128 murders already this year, is on pace to reach the city's highest murder rate in a decade.