The States can’t determine who qualifies to run for President. The constitution alone states the criteria
to be eligible. If a Governor is idiotic enough to sign a law requiring a Presidential candidate to provide
tax returns in order to be on the ballot, he/she would be a darling of the media until SCOTUS kicked him/her
in the teeth.
Far lefty Jerry Brown (who didn’t release his returns

) vetoed such a bill in 2017. Newsom has Oval
Office ambitions, so I don’t think he'd be willing to suffer a SCOTUS rebuke over the new bill currently
floating in the CA legislature.
“While I recognize the political attractiveness – even the merits – of getting President Trump’s tax returns,
I worry about the political perils of individual states seeking to regulate presidential elections in this manner,”
Brown wrote in a veto message. “First, it may not be constitutional. Second, it sets a ‘slippery slope’ precedent.
Today we require tax returns, but what would be next? Five years of health records? A certified birth certificate?
High school report cards? And will these requirements vary depending on which political party is in power?”
https://www.politico.com/story/2017/10/16/jerry-brown-trump-tax-returns-bill-243799