Gov. Gavin Newsom and state Atty.Gen. Xavier Becerra announced the legal action at a Capitol news conference just hours after President Trump declared a national emergency in an attempt to divert up to $6.6 billion from other projects.
The Trump administration has repeatedly issued legal waivers to speed construction of a border wall, bypassing environmental laws.
The Supreme Court will rule on the plan to add a citizenship question to the 2020 census, dealing a blow to California and other states with large numbers of immigrants.