Can a city prohibit political signs but allow commercial signs, such as ‘for sale’ signs?
No, the First Amendment, if anything, provides greater protection for political speech than commercial speech. Political speech is said to represent the core values that the First Amendment was designed to protect. The Supreme Court has created what has come to be known as the commercial-speech doctrine, saying that commercial speech represents a subordinate place in First Amendment jurisprudence. For example, content-based restrictions on political speech are subject to the highest form of judicial review, called strict scrutiny. On the other hand, content-based restrictions on commercial speech are subject to a lesser form of review, called intermediate scrutiny. What this legalese means, in essence, is that yard signs on political topics are entitled to at least as much (and probably more) protection as commercial signs.