We remember in one of our previous lifetimes when we were helping candidates in their races that when it got to that point, some campaign workers were not above erecting their candidates' signs above the existing ones. You weren't defacing their signs or obstructing the view, so it seemed like a fair thing to do and it worked.
And it also keeps the political signs in one place instead of stringing them along the right-of-way. We'll see whether the SGP campaign staff heeds the advice.)