California's governor Tuesday signed a law that would prevent President Donald Trump appearing on the state's primary ballot unless he discloses his income tax returns.
While there is little sign of any Republican mounting a serious challenge to Trump's re-election bid at the primaries, Democrats are seeking to force the US president to hand over his financial documents.
"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," said Democratic Governor Gavin Newsom in a statement as he signed the bill.
The law which took immediate effect on Tuesday does not mention Trump by name.
But the US president famously broke with tradition and refused to release his tax returns when he ran in 2016.
The legislation comes at a time when Democratic lawmakers have opened investigations focused on Trump's tax returns as well as matters related to Russian efforts to sway the 2016 election.
"The disclosure required by this bill will shed light on conflicts of interest, self-dealing, or influence from domestic and foreign business interest," said Newsom.
The new California law means any candidate for US president must file five years' worth of tax returns in order to contest primary elections in the country's most populous state.
It was passed by California's heavily Democratic legislature last month, and appears only to affect candidates running in their parties' primary votes, not the November 2020 poll itself.
But in an April survey indicating that others could be mulling parallel moves, the National Conference of State Legislatures found that 17 state legislatures had already introduced similar new bills this year.
Trump's campaign staff said state laws on presidential candidates' eligibility were unconstitutional.
"The Constitution is clear on the qualifications for someone to serve as president and states cannot add additional requirements on their own," Tim Murtaugh, communications director for Trump's reelection campaign, said in a statement to the LA Times.
Jay Sekulow, counsel to the president, said California's law "will be answered in court," according to US media reports.
(Source: AFP)