WASHINGTON ― President Donald Trump broke with 40 years of precedent in the 2016 presidential election when he refused to voluntarily disclose his tax returns to the public. State lawmakers are taking steps to make sure that doesn’t happen again when he runs for re-election.

Lawmakers in 26 states have introduced legislation to require presidential and vice presidential candidates to publicly disclose their tax returns to get on the ballot. If future candidates should fail to release their tax returns under this legislation, they would not be allowed to appear on the ballot.

Trump claims that he can’t release his tax returns because they are under a routine audit ― an assertion that is false. President Richard Nixon, the first to disclose his tax returns, released them in 1973 even though they were under audit at the time.

Trump’s refusal to disclose his tax returns has only increased speculation about what could be in them. His returns would detail the amount he paid in taxes, how much he donated to charity, whether he paid foreign taxes, whether he has foreign bank accounts and the profits and losses of his many businesses.

Two pages of Trump’s 2005 tax returns were disclosed to journalist David Cay Johnston, which did nothing to stymie public interest in the issue. Thousands of people are expected to protest in dozens of cities across the country on April 15 to demand Trump release his tax returns.

While state lawmakers are seeking to force Trump to disclose his tax returns, Republicans in Congress have blocked three of Democrats’ amendments to obtain them.

States Where Presidential Tax Return Bills Have Been Introduced

DLCC The states in blue have introduced legislation to require presidential candidates to disclose their tax returns in order to gain ballot access in 2020.

Democratic New York state Sen. Brad Hoylman introduced the first legislation to make Trump disclose his tax returns to get on the 2020 New York state ballot. His bill ― the Tax Returns Uniformly Made Public, or TRUMP ― would require all presidential and vice presidential candidates to release the past five years of their taxes 60 days prior to the election. If a candidate failed to meet this requirement, they would be excluded from the ballot.

“We thought, given the barriers in Washington, that states could take up the effort to require presidential and vice presidential candidates to release their taxes in order to qualify for the ballot,” Hoylman told The Huffington Post.

He added, “Ballot qualifications are a time-honored responsibility of states, and at the same time, the current president had broken with over four decades of political tradition.”

Lawmakers in another 25 states have introduced nearly identical legislation since Hoylman first released his bill in December. These include Democratic strongholds and states Trump won.

In New Jersey, legislation to force Trump to disclose his tax returns has passed both chambers of the legislature with support from both Democrats and Republicans. It faces a likely veto from Gov. Chris Christie (R), a Trump ally. But Christie’s term in office will end in 2017 and he’s likely to be replaced by a Democrat. That means there is higher likelihood the state could approve this bill in 2018.

New Jersey is not the only state in which Republicans have voted for or sponsored this type of legislation. In Georgia, New Mexico and Minnesota, Republican lawmakers have introduced bills to require candidates to disclose their tax returns to gain ballot access.

Alex Milan Tracy/Anadolu Agency/Getty Images A woman holds up a sign made of tax returns in Portland, Oregon, on Feb. 28, 2017.

Groups supporting the disclosure legislation emphasize that it is not just about Trump, but any future candidate, now that the current president has broken the 40-year norm.

“This is more than just about Donald Trump,” Aaron Scherb, the director of legislative affairs at the nonpartisan nonprofit Common Cause, told HuffPost. “This is about ensuring the American public can have a full understanding of the presidential candidates moving forward, to make sure that any current or future presidents do not have conflicts of interest.”

There is, however, the question of the legality of forcing candidates to disclose their tax returns to gain ballot access. Rick Hasen, an election law expert at University of California, Irvine School of Law, argued in Politico that one legal argument that would allow such laws is the Supreme Court’s 2000 Bush v. Gore decision.

Hasen writes, “the Court wrote that even though state legislators have given each state’s voters the right to vote for presidential electors, at any time a state legislature can ‘take back the power’ to appoint electors.”

This means that states could theoretically change the rules about how members of the Electoral College can or cannot vote as they see fit. If a state wanted to take away the power from electors to vote for president and give that power to the state legislature, that would be allowed. At least, so goes an argument that has never faced further constitutional scrutiny outside of the supposed one-off case of Bush v. Gore.

But Hasen further argues that Republicans could retaliate with their own ballot access rules to negatively target Democratic candidates.

“Democrats should consider the Pandora’s box they might be opening here,” Hasen writes. “Will solidly Republican states allow electors to vote only for Republican candidates for president? If the tax gambit is OK, then such a law might also be constitutional.”

“I say bring it on, if it results in voters having informed choices in the next election,” Hoylman said about possible retaliatory legislation.