Viewpoint discrimination is a form of content discrimination particularly disfavored by the courts. When the government engages in content discrimination, it is restricting speech on a given subject matter. When it engages in viewpoint discrimination, it is singling out a particular opinion or perspective on that subject matter for treatment unlike that given to other viewpoints.
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Because the government is essentially taking sides in a debate when it engages in viewpoint discrimination, the Supreme Court has held viewpoint-based restrictions to be especially offensive to the First Amendment. Such restrictions are treated as presumptively unconstitutional.
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A good example is Congregation Lubavitch v. City of Cincinnati (6th Cir. 1993), which dealt with expressive access to Fountain Square, a public square in downtown Cincinnati.
For many years, the city had allowed a broad range of private groups to erect expressive signs and exhibits on the square and to leave them there for short periods of time. But the city was not so agreeable when it was approached, separately and persistently, by a Jewish congregation and the Ku Klux Klan, both of which sought to erect overnight displays on Fountain Square.
Instead of granting these requests, the city quickly enacted a new ordinance that banned all overnight displays on the square, except those sponsored or co-sponsored by the city. Thus the new ordinance gave the city complete freedom to discriminate between favored groups — such as the Kiwanis Club, an Oktoberfest committee, and a librarians’ organization — and disfavored groups — such as the Jewish congregation and the Klan.
Because the city was using its new ordinance to invite or exclude each group based on its identity and message, the city was engaged in viewpoint discrimination.