The Court just agreed to hear Town of Greece v. Galloway, a case involving legislative prayer. In Marsh v. Chambers (1983), the Supreme Court upheld legislative prayers against an Establishment Clause challenge, based on the very long American tradition of such prayers (dating back to the same First Congress that proposed the Establishment Clause); nonetheless, the scope of Marsh is unclear, and in particular it’s unclear to what extent legislative prayers might be seen as unconstitutionally preferring a particular religion or denomination.
Or that at least is the narrow question raised by the case. But I think it’s also possible that the Court may use the case as a means of reconsidering the “endorsement test,” under which the Establishment Clause is read as barring government speech (or even government action) that a “reasonable observer” would see as “endorsing or disapproving” of religion (either a particular religion or religion generally). The test has long been controversial; it was relied on by the decision below, so it’s very much in play in this case; and I suspect that there are five votes to overrule it. (Justices Kennedy, Scalia, and Thomas are on the record as rejecting it, and I suspect Chief Justice Roberts and Justice Alito take a similar view.) Should be a very interesting decision, which will be out in the first half of next year.