Police say a petrol bomb was thrown through the window of the office in the town of Hillsborough and the words "Nazi Republicans leave town or else" were spray-painted on a nearby building.

The walls of the small office were blackened and a sofa was burned down to its springs before the fire went out, but no-one was hurt.

State GOP executive director Dallas Woodhouse said people sometimes work after-hours, and he felt lucky that no one was there at the time.

He said the bottle appeared to have landed on or near the sofa where volunteers sometimes take naps.

"They are working around the clock. It is a miracle that nobody was killed," he said in an interview, calling the fire "political terrorism".

We don't know who is responsible , but they clearly hate our candidates, volunteers, staff and Constitution @NCGOP pic.twitter.com/81xBx6ex4t

— Dallas Woodhouse (@DallasWoodhouse) October 16, 2016

Hillary Clinton tweeted the attack was "horrific and unacceptable", adding she was grateful nobody had been hurt.

Donald Trump tweeted that the attack was carried out by "animals representing Hilary Clinton" because "we are winning".

He later tweeted support for Orange County party members: "With you all the way, will never forget. Now we have to win. Proud of you all!"

Animals representing Hillary Clinton and Dems in North Carolina just firebombed our office in Orange County because we are winning @NCGOP

— Donald J. Trump (@realDonaldTrump) October 16, 2016

Orange County GOP chairman Daniel Ashley said he was unaware of anyone making violent threats against the office.

Hillsborough mayor and Democrat Tom Stevens said it was lucky the fire did not spread to nearby building, adding the act did not represent the character of Orange County, where registered Democrats outnumber Republicans by three to one.

"I'd like to believe we aspire to respect hearing differing views," Mr Stevens said in an interview. "This is very troubling."