A Republican Party office in North Carolina has been damaged by fire, and a nearby building spray-painted with an anti-GOP slogan, authorities say.
A bottle filled with flammable liquid was thrown through the window of the Orange County Republican Party headquarters overnight, the party said.
State GOP director Dallas Woodhouse said no-one was injured, but a security alert has been sent to other offices.
“Nazi Republicans get out of town or else,” was written on the building.
Democratic presidential candidate Hillary Clinton said in a tweet the attack was “horrific and unacceptable”.
But the Republican nominee, Donald Trump, appeared to blame Democrat supporters for the attack.
“Animals representing Hillary Clinton and Dems in North Carolina just firebombed our office in Orange County because we are winning @NCGOP,” he said on Twitter.
Local police said they were investigating the incident alongside the Bureau of Alcohol, Tobacco, Firearms and Explosives.
Tom Stevens, the Democrat mayor of the town, Hillsborough, condemned the act in a statement.
“This highly disturbing act goes far beyond vandalising property; it wilfully threatens our community’s safety via fire, and its hateful message undermines decency, respect and integrity in civic participation,” he said.
US election: Republican office in North Carolina vandalised