I'll give you the best example I can think of without, I'm afraid, any links or pages to click, I don't know what you need in the way of proof, but I think the best example of political art making change was the nineteen sixties American tee-shirt.
I don't have time this morning to give you the full picture as I saw it, but prior to 1960 the tee-shirt was one thing, white and something you wore under a dress shirt. Wheaties and General Mills changed that by issuing a Breakfast of Champions tee-shirt in the late fifties. (It could have been earlier, but I don't think so.) Followed by Disney with their Davy Crockett, King of the Wild Frontier shirts. Other advertisers followed and soon the white tee-shirt was a movable billboard.
Click over to the anti-nuclear submarine protests in 1960 in Connecticut, the police said the protesters couldn't carry signs because they could be used as weapons, so the anti-nuke printed their

on tee-shirts. Movable speech.
From then on there wasn't a speech or a rally or a protest that didn't have an official unofficial tee-shirt.
During the protests against the War in VietNam the tee-shirt grew from just something one wore at a protest, but in one's personal life. It was instant identification for friends and opponents alike. Tee-shirts were the easiest, most productive purveyors of the counterculture message: Stop the War, Stop the Bombing, Make Love, Not War

, Give Peace a Chance,
the Woodstock Nation Bird on the guitar

, you could walk down Main Street and fight the power.
Now, do I have proof that any of this changed a vote or saved a GI from dying? Nope.
Joe(Just food for thought)Nation