California is facing a second legal challenge to its new law requiring presidential candidates to release their tax returns.

Judicial Watch Inc.'s lawsuit in federal court in Sacramento seeks to block the law signed by Gov. Gavin Newsom on July 30, requiring five years of tax returns for presidential candidates to access the state primary ballot. Republican presidential candidate Roque “Rocky” De La Fuente of San Diego filed a similar suit July 30.

Judicial Watch claims the suit improperly burdens voters’ rights and oversteps the state’s role in federal elections.

All 50 states could end up with...