Washington (CNN) President Donald Trump has made pretty clear he's not going to release his tax returns, but that hasn't stopped Democrats from scheming ways to make him do it.

They thought they had a winning idea in California -- just require public release of tax returns as a prerequisite for getting on the 2020 ballot. ( Similar efforts are underway in other states They've been vetoed by other governors in the past .)

The bill, passed by Democratic majorities in the state legislature, would have required any presidential candidate to publicly release five years of tax returns in order to be eligible for the primary ballot.

But California Gov. Jerry Brown, himself a Democrat, dashed those hopes (at least for the California version) late Sunday when he vetoed the bill.

Why?

In a veto message , Brown said the tax return requirement goes too far. Here's the meat of his reasoning:

"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. 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?"

"A qualified candidate's ability to appear on the ballot is fundamental to our democratic system. For that reason, I hesitate to start down a road that well might lead to an ever escalating set of differing state requirements for presidential candidates."

Brown has a point about the constitutional issues with the law. It would certainly have faced court challenges between now and the next presidential primary in 2020.

It should also be pointed out that Brown did not release his own tax returns when he ran for a third term as governor in 2010. His first two terms were between 1975 and 1983. Politifact points out he did not release his tax returns as a presidential candidate in 1992.

It's hard to understate exactly how politically attractive Trump's tax returns are to Democrats. While it is clear from what's known (and his own admission) that he tried very hard to lower his tax bill, what exactly is in the returns, with some glaring exceptions, has remained a tantalizing mystery.

On that primary, Brown did recently sign a bill to move the California primary much earlier in the year in 2020 -- to March. That could give Golden State and its many, many delegates a much more vocal role earlier in the process. They are still smarting that their primary in 2016 came in June, long after the Republican nomination was locked up by Trump and the day after Hillary Clinton locked up the Democratic nomination.

Brown is also in the midst of dealing with devastating wildfires that have ravaged parts of Northern California and for which federal aid will be very important.

The tax return issue isn't going away as Trump pushes tax reform

Trump has long said he would release the tax returns once he was no longer under IRS audit. The details of that audit have not been made public. Since his election, the argument of Trump and his aides has distilled to, essentially, he got elected, so voters don't care.

But the issue of his tax returns won't be going away anytime soon, particularly as Trump and Republicans on Capitol Hill turn to a massive tax-reform proposal. Tax reform is important to Republicans for two key reasons, in this order:

They truly and legitimately believe the tax system needs to be reformed and this is the strongest the party has been in Washington for a really long time. After flailing on his pledge to repeal Obamacare, Trump needs to get a policy win. Any policy win will do.

But as Republicans push a tax system that would lower tax rates for the highest earners, it's clear that if Trump didn't erase most of his tax bill with write downs spread over many years, his own plan could give him a big tax cut.

Could, but we don't really know, since he's kept his returns a secret.

He said in May he would not release his tax returns as part of a deal with Democrats to enact tax reform.

"As you know, I'm under routine audit, so they're not going to be done. But you know, at a certain point, that's something I will consider," he told the Economist in May . "But I would never consider it as part of a deal. I think that would be unfair to the deal. It would be disrespectful of the importance of this deal."

In the same interview, Trump said he's very proud of his tax returns, actually, and he might release them when he's out of office.

Republicans hope they don't need Democratic votes for the bulk of their tax plan, which they are scheming to squeeze through Congress and around normal rules by passing it as part of a budget process that skirts the filibuster.

"It's going to be much harder to get tax reform done if the President doesn't disclose his taxes," Senate Minority Leader Chuck Schumer has said. "For the very simple reason that when there is a provision in the bill, people are going to say, 'Oh, this is for President Trump and his business, not for the benefit of the American people.' Particularly when you have the sort of large real estate business that he has. There are all kinds of tax laws that affect it. If the President is interested in tax reform, he should release his tax returns because it's going to make it much harder to pass it without it."

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Trump has promised that his tax reform proposal won't help him. But we have to take his word for it. His top economic adviser, Gary Cohn, wouldn't make a similar pledge when asked repeatedly about, instead arguing Americans are worried about their own financial situations.