California has passed an unprecedented requirement that Donald Trump release his tax returns, or else find himself ineligible for inclusion on the state’s primary ballot next year.

The law was signed on Tuesday by governor Gavin Newsom, and requires all presidential candidates to submit five years of income tax filings in order to be eligible for the primary ballot.

The bill was originally passed in the state legislature on a party line vote.

“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,” Mr Newsom said in a statement.

He continued: “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. The disclosure required by this bill will shed light on conflicts of interest, self-dealing, or influence from domestic and foreign business interests.”

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The bill marks the latest effort by Democrats across the country to force Mr Trump to release his tax returns, including an effort in the House of Representatives and in New York State.

Those efforts have been thwarted by Mr Trump, who broke from modern day campaigning tradition by refusing to offer up his tax returns while running for president in 2016.

Since then, questions have swirled about why the president would decline to hand over those documents, even as he has claimed he is held back from making the disclosures because of tax audits.

Among the concerns raised by Mr Trump’s political opponents are whether he or his companies have ongoing business relationships or loans from foreign entities, and whether Mr Trump might have conflicts-of-interest in his official work as president stemming from his private financial dealings.

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But, Republicans have cast the effort in California as an unconstitutional overreach by a party hell bent on making life miserable for Mr Trump.

“The Constitution is clear on the qualifications for someone to serve as president and states cannot add additional requirements on their own,” Tim Murtaugh, the communications director for Mr Trump’s re-election campaign, said in a statement provided to the Los Angeles Times.