California's governor has signed a law that would prevent US President Donald Trump from 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 Mr Trump's re-election bid in 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 yesterday, does not mention Mr 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 politicians have opened investigations focused on Mr 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 Mr Newsom. 

The 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.

The bill was passed by California's heavily Democratic legislature last month.

It 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. 

In seven of those states, decisions are currently pending, including New York, New Jersey and Illinois.

A similar attempt in California in 2017 was vetoed by then-governor Jerry Brown, another Democrat. He expressed concern about the precedent that setting requirements for candidates could create.

Mr 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 Mr Trump's re-election 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.

But Harvard law professor and constitutional scholar Laurence Tribe said the bill "should survive the predictable constitutional challenge".

"California isn't adding any requirements for the presidency - which it couldn't do - but just ensuring that its voters are fully informed about all aspirants," he tweeted.