Tax filings, which will redact private information, must be submitted for disclosure by November to gain a spot on California's presidential primary ballot in March, the LA Times reported. But the bill was never brought up for a vote and died in committee last month.
While the law is aimed at Trump, it would apply to all presidential contenders and candidates for governor.
"These are extraordinary times and states have a legal and moral duty to do everything in their power to ensure leaders seeking the highest offices meet minimal standards, and to restore public confidence", Newsom said in his statement.
"There are very good reasons why the very liberal Gov". Brown wrote in his veto message. "What's next, five years of health records?"
The courts will likely have the final say.
The bill was signed into law almost two years after then-California Gov. "The bill also violates the 1st Amendment right of association since California can't tell political parties which candidates their members can or cannot vote for in a primary election".
"As one of the largest economies in the world and home to one in nine Americans eligible to vote, California has a special responsibility to require this information of presidential and gubernatorial candidates", Newsom continued, adding that the law is constitutional because "the United States Constitution grants states the authority to determine how their electors are chosen". Those qualifications include the requirement that presidential candidates be over age 35.
The U.S. Supreme Court has previously halted state efforts to add ballot access rules for congressional candidates.
Last week, Trump sued to stop the House of Representatives from obtaining a copy of his NY state returns, made available under a bill signed by Gov. Andrew Cuomo earlier this month. Jerry Brown vetoed this bill two years ago - it's unconstitutional and it opens up the possibility for states to load up more requirements on candidates in future elections.
The measure is a direct challenge to Trump, who has bucked tradition by not releasing any of his tax returns, prompting concerns from Democrats and others about potential conflicts of interest.
In a tweet to Trump, McGuire said: "If you want to be on the CA primary ballot, release your returns".
The law, now titled the "Presidential Tax Transparency and Accountability Act", demands that presidential candidates disclose five years of federal tax returns, in order to be included on the state's primary election ballot. The returns will then be posted online.