The Kalamunda MLA’s posters from Darlington to Mundaring have been the target of graffiti.
Mr Day acknowledged the preference deal with One Nation had put the WA Liberal Party in a difficult position.
“I wasn’t involved in the decision to do a deal with One Nation but there are people in the Hills community who are supportive of Pauline Hanson,” he said.
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The Liberal candidate for Midland, Daniel Parasiliti, said he was disappointed his election posters had also been targeted by vandals.
Mr Parasiliti said on his drive to work in Midland early last Friday morning he saw every one of his posters had been snapped, broken, graffitied or stolen.
“Even my large banner on the corner of Morrison Road and Great Eastern Highway has been stolen,” he said.
“My advertising signs have been snatched from inside people’s private property.
“That is very poor form for such blatant theft and vandalism to occur.
“Whoever did this would have trespassed on private property to remove the signs from people’s frontyards.
“It is disappointing because I have permission from my supporters to erect my signs.”
Mr Parasiliti said while the vandalism was disrespectful, it would not deter him ahead of March 11.
“My signs may get knocked down, but I’ll get them up again,” he said.