That's because the proposed law has no direct precedent. Dozens of bills with similar tax return requirements were introduced earlier this year around the country, but none was signed into law.
Legal experts are split on the constitutionality of the requirement. At issue is whether the tax return requirement is a procedural hurdle or a brand new qualification for office.
The U.S. Supreme Court has allowed states to tack on requirements like filing fees, but not to change the fundamental threshold for a presidential candidate: a citizen who is at least 35 years old, and who has lived in the country for at least 14 years.
"The idea that this burdens a class of people doesn’t really sing to me," said Levinson. "It looks more like one of those procedural rules in terms of how you obtain access."
Critics of SB 149, argue that the measure is merely a new barrier erected by Democrats for political gain.
They also point to high court decisions barring the requirement of some types of information from candidates.
In 1964, the Supreme Court ruled in Anderson v. Martin that Louisiana had violated the constitution when it listed the race of candidates next to their names on the ballot.
"It might be a truthful piece of information that voters would like to know for some candidates, but you can’t put a thumb on the scale against candidates who you disfavor," said Derek Muller, a professor at Pepperdine University School of Law.
Others argue that unlike race or age, the decision to release tax returns is something candidates can proactively control.
"The proposed laws mandate transparency rather than interposing obstacles that some would-be candidates cannot surmount," wrote Harvard Law School professor Laurence H. Tribe, in support of the bill.
SB 149, carried by Senator Mike McGuire, D-Healdsburg, passed the Assembly and Senate last week, with the support of just two Republicans.
As the bill moved through the Legislature, opponents eschewed discussion of the legal standard in favor of discussing a perceived double standard.
"If this bill included state legislators, I could support it, because it wouldn’t be looking so much like it’s grandstanding," said Senator Joel Anderson, R-San Diego.
In an interview with CNN this week, Gov. Brown was asked if he could sign the bill without having released his own returns in 2010 and 2014 gubernatorial races.
"I did release them in my earlier campaigns," Brown replied, in reference to his previous runs for governor and president. "This time around, my opponents wouldn’t do it, so I didn’t either."
Brown said he would weigh the legal arguments around the bill, and release his decision "very soon."