The overlap between people who would put these signs up and the people who shoot other people merely for appearing on their doorbell cameras is pretty wide.
I support but do not recommend this type of vandalism.
After seeing two of my neighbors put up Trump signs in 2016, I said "no fucking way are we ever putting a political sign on our lawn." I'm not going to risk anything around these people. Who knows how insane they are? I don't even want a knock on my door and a lecture.
Next fill a bunch of trash cans with cheap gas, haul it around with you and sell it door to door as a distraction while your friend is switching the signs. Wild card, bitches!
I don't much care about the legality. I'd be much more concerned about a Trump supporter sitting in front of a monitor staring at a hidden camera feed of the sign on his lawn, just itching for someone to vandalize it so he could come out guns blazing,.
reminder that conservative are more frequently blue collar, more frequently rural, and more frequently working labor of some kind, the likelihood that they have security cameras is low, and the likelihood that they work is even lower.
while they likely do have guns, and are more likely to use them, i doubt that it makes a significant difference if you plan properly.
and it also doesn't take much to completely bonk the setup, or better yet, just assume it isn't working. There's a good chance that 75% of the reason security cameras exist is actually for deterrence.