No arrests have been made.
Gov. Pat McCrory on Monday called the fire an "assault on our democracy."
Speaking from the scene, McCrory said the incident won't get in the way of fair elections.
Hillsborough police said they are still processing evidence from the scene and they are receiving assistance from the FBI, North Carolina State Bureau of Investigations and the Bureau of Alcohol, Tobacco, Firearms and Explosives.
A bottle of flammable liquid was thrown through the Orange County Republican headquarters, police said
"The flammable substance appears to have ignited inside the building, burned some furniture and damaged the building's interior before going out. The substance was housed in a bottle thrown through one of the building's front windows," according to a statement by the town of Hillsborough.
A 911 call was made by the owner of a business nearby, the town said.
"Someone has firebombed through the window of the Republican Party next to me and sprayed all over the side of my building, 'Nazi Republicans leave town or else,' " the caller said.
The caller said: "They threw some sort of firebomb through the window because it smoked up under the porch and all around the window frame. ... I can smell smoke, so obviously it went out on its own or didn't work well, but it is some sort of incendiary device."
Hillsborough is about 10 miles northwest of Durham.
North Carolina is a key swing state in the presidential election between Hillary Clinton and Donald Trump.
Early Sunday evening, the Clinton campaign tweeted a note of sympathy.
"The attack on the Orange County HQ @NCGOP office is horrific and unacceptable. Very grateful that everyone is safe."
"The office itself is a total loss," Dallas Woodhouse, executive director of the state Republican Party, told The Charlotte Observer
. "The only thing important to us is that nobody was killed, and they very well could have been."
The weekend firebombing episode become fodder for claims by Trump that the election is "rigged" in Clinton's favor, reflecting the intensity surrounding the campaign in the final weeks of the race. In the evening, Trump -- without evidence -- blamed the firebombing on Clinton supporters.
"Animals representing Hillary Clinton and Dems in North Carolina just firebombed our office in Orange County because we are winning @NCGOP," Trump tweeted Sunday night.
Woodhouse told CNN's John Berman and Kate Bolduan that roughly 80 county GOP offices were told to close early Sunday, and a temporary mobile office was erected in Orange County, near the site of the firebombing.
A group of Democrats started a GoFundMe fundraising page to help with rebuilding the office. The campaign has accumulated
more than $13,000.
"We've had a lot of people reach out. We've had a lot of people who aren't political who are at the office volunteering," Woodhouse said. "We have heard from some Democrat groups. I agree with both Ms. Clinton and Mr. Trump that these were animals who did this, no matter who they represent, and it's disgusting and unacceptable.
Democratic office vandalized
Fifteen miles south, in Carrboro, a Democratic office was spray-painted with the words "Death to Capitalism."
Orange County Democratic Party Chairman Matt Hughes says the office was closed midday Sunday after employees discovered the scrawl.
Capt. Chris Atack of the Carrboro Police Department told CNN in an email that the graffiti is consistent in theme with others officials have seen in the downtown areas for years.
He said the incident was not believed to be related to the GOP vandalism in Hillsborough.
"We have no information suggesting that this is in any way related to the proximity to Democratic campaign headquarters or events in Hillsborough," Atak said.
Hughes, the Democratic Party chairman, says it is "a heck of a coincidence."