You'd need to check with whatever township it's in. Like here's East Bradford's policy.

https://eastbradford.org/173/Political-Signs

If it's on public property you could always report your neighbor for littering; take a picture or it'd be a he-said-she-said situation though and probably would only end up in hard feelings for both of you and police grumbling about it being something they can't do anything about without hard proof.