Paint was daubed on the Lurgan constituency office of the UUP leader Mr David Trimble overnight causing minor damage.
Less than 24 hours after a street altercation with the Rev Ian Paisley's Democratic Unionists, red and white paint was smeared on the front door of the Upper Bann office in Lurgan, Co Armagh.
Mr Trimble was not in the premises at the time.
A spokesman for the Ulster Unionist Party leader did not accuse the DUP of any involvement. But he added: "It seems a bit strange that a day after Mr Trimble takes on the DUP outside his party HQ, something like this happens."
Mr Trimble had been involved in a furious head-to-head slanging match with Mr Paisley's deputy, the east Belfast MP Peter Robinson.
The row plunged relations between the two sides to an all-time low as the DUP bids to become the single largest Unionist party in next Wednesday elections to the Northern Ireland Assembly.
Mr Trimble said the attack had been carried out by "arrogant bully boys", but he would continue to canvas the constituency.
"The language used by the DUP members yesterday to my not being welcome in my constituency does a great disservice to the people of Upper Bann and Portadown in particular."