For nearly two years, magazines, newspapers and other media groups facing defamation lawsuits in Washington have had a new weapon in their litigation arsenal: the city’s law barring strategic lawsuits against public participation, or SLAPPs.

As the anti-SLAPP law’s second anniversary approaches at the end of March, though, its fate is unclear. A case before the U.S. Court of Appeals for the D.C. Circuit next month could answer critical unresolved questions — notably, whether the law applies in federal court.