It’s not unusual to see ads on city buses in Portland, promoting everything from banks to cellphone providers to new department stores. But it is rare for a bus advertising campaign itself to become the focus of news coverage.

That’s because the posters in question ask Portland voters to support a referendum instructing city police not to arrest people over 21 for the possession and use of marijuana. Fearing that the campaign could encourage teenage drug use, anti-drug abuse advocates are urging Metro to ban marijuana advertising on its buses and in its bus shelters.

But you don’t have to support legalizing marijuana in Portland – or anywhere else – to realize that the critics’ effort is a nonstarter. The Marijuana Policy Project’s pro-legalization ad campaign is political speech, intended to rally public support for an issue. Those who oppose legalizing recreational marijuana use in Portland have the same constitutionally protected opportunity to make their own views known. They’d be better off presenting their case to the public, rather than trying to squelch speech they don’t agree with.

The Marijuana Policy Project’s political posters feature the same clean-cut, well-dressed adults familiar to any American media consumer. Each ad shows a different person saying why he or she prefers marijuana over alcohol. But 21 Reasons, which seeks to reduce drug use by young people, doesn’t find this message to be a positive one. In fact, the anti-drug abuse group worries that by reducing the perceived risk, the bus ads will lead more minors to use drugs.

Marijuana use by minors is a concern. However, there are other ways to prevent adolescent drug use than blocking information that can be interpreted as encouraging marijuana usage. According to a report by the office of the U.S. drug czar, for example, parental involvement and limit-setting are key to keeping kids drug-free.

Ultimately, though, the issues raised by 21 Reasons are beside the point. The Marijuana Policy Project isn’t selling a drug or promoting its use; it’s asking voters to change a law. Political speech is among the most protected speech there is, and barring these or any other political messages from Metro buses would erode these crucial protections.

What’s more, the bus line is owned by the city of Portland and overseen by a board of directors from member communities, so pulling the pro-legalization ads from city buses would be tantamount to these municipalities’ taking an official stand on a citizen referendum – a highly inappropriate move.

Opponents of the pro-legalization ads have their own options under the First Amendment. They can start an ad campaign of their own, post to blogs and write letters to the editor. Then their views will become part of the public debate, which is where they belong. Everybody who feels strongly about an issue should be seeking access to the democratic dialogue, not trying to keep other people out of it.