Vandals have scrawled “Vote UKIP” on a listed building at Cogges Manor Farm.
The graffiti, pictured, was discovered at the museum over the weekend and was yesterday being washed off.
Local councillors Jeanette Baker and Duncan Enright said they were shocked by the act of vandalism.
It happened more than a week after local and European elections, in which UKIP failed to win any seats in Oxfordshire.
Call police on 101 with any information about the graffiti.
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