California Attorney General Rob Bonta slammed the letter.
Millions more Americans might qualify for dual Canadian citizenship under a recent change to Canada's requirements.
As President Trump looked on during an unprecedented visit to the Supreme Court, a majority of justices appeared skeptical of his administration's bid to end birthright citizenship.
The Bay Area has a deep connection to birthright citizenship. It's a landmark case that affirmed the right began in San Francisco.
The bill, would require Americans to prove they are citizens when they register to vote, mostly through a valid U.S. passport or birth certificate.
A judge permanently blocked two provisions of the executive order that sought to impose proof-of-citizenship rules.