Petrol was poured through the shutters of the Alliance party office in Lisburn in an attempted arson attack on Saturday morning.
The incident, at around 6am at the party’s Lagan Valley constituency office in Graham Gardens in the city, is the latest in a number of attacks on party offices in recent months.
Lagan Valley Assembly member Trevor Lunn said it was an attack on democracy.
“There is no place in our society for this violence that should be condemned by all,” said Mr Lunn.
“This attack will not deter us from continuing to work on behalf of the people of Lagan Valley. This is the latest in a number of attacks on Alliance Party offices, but we will not give in to violence and intimidation. Alliance will continue to work to achieve a shared future for everyone.
Police said a member of the public reported hearing a bang in the area and that follow-up investigations revealed fire damage behind the shutters of the office.
A forensic examination is underway.
A police spokesman said: “At this stage it is not thought that an explosive device was used.
“Police are keen to speak to a woman who was walking a dog in the area at the time.”
Secretary of State Theresa Villers condemned the attack, saying it was unacceptable.
“I condemn this attack, as I have others which have targeted the Alliance offices over recent months,” said Ms Villiers.
“It is totally unacceptable for elected representatives and their staff to be subject to this kind of intimidation.
“I would appeal to anyone with information on those responsible to pass it to the PSNI as soon as possible.
“I extend my support and sympathy to David Ford and his staff and colleagues.”