New York's attorney general has disclosed that his office is investigating Donald Trump's charity to determine whether it has abided by state laws governing nonprofits.

The disclosure came on a day Democrats in the US House of Representatives called for a federal investigation into a donation the Trump Foundation made to a political group supporting Florida's attorney general after her office said it was considering legal action against Trump University.

"My interest in this issue really is in my capacity as regulator of non-profits in New York state," Eric Schneiderman, the New York attorney general, told CNN on Tuesday.

"We have been concerned that the Trump Foundation has engaged in impropriety from that point of view."

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Schneiderman has been at loggerheads with the Republican presidential nominee over the Trump University real estate programme, which he has called "straight up fraud".

The Trump Foundation has faced a series of damaging stories, including by The Washington Post, which reported over the weekend that the White House candidate himself had not donated to his own charity since 2008.

The newspaper found other irregularities, including that Trump spent $20,000 of money that had been set aside for charitable purposes to purchase a 6ft painting of himself.

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Trump Foundation made an illegal $25,000 donation to a campaign group affiliated with Pam Bondi, Florida's attorney general, in 2013 as she was considering joining Schneiderman's fraud case against Trump University.

"We've inquired into it and we've had correspondence with them. I didn't make a big deal out of it or hold a press conference," Schneiderman said.  

"But we have been looking into the Trump Foundation to make sure it's complying with the laws governing charities in New York."

Democrats on the House Judiciary Committee issued an open letter asking US Attorney General Loretta Lynch to investigate the donation to Bondi.

"After receiving these funds, Mrs Bondi declined to further investigate Mr Trump's business interests. This fact pattern indicates that these payments may have influenced Mrs Bondi's official decision not to participate in litigation against Mr Trump," the letter read.

"A number of criminal statutes would appear to be implicated by this course of conduct."

Trump, in turn, has criticised Schneiderman personally, calling him "Clockwork Eric".

'Deplorable' supporters

For her part, Hillary Clinton, the Democratic nominee, stirred up a storm on Friday when she called half of Trump's supporters "deplorables".

Clinton returns on Thursday to the White House campaign fray after a few days at home recovering from pneumonia in a health scare.

Clinton's stumble, captured on amateur video and seen online and on television, gave Trump a new opening to question her fitness for the nation's highest office as the race intensifies.  

"While my opponent slanders you as deplorable and irredeemable, I call you hardworking American patriots who love your country and want a better future for all of our people," Trump said in Iowa.

Obama campaigned for Clinton in Philadelphia on Tuesday by giving a speech [Tracie Van Auken/EPA]

Clinton's campaign initially said thta she had been suffering the ill-effects of dehydration and "overheating".

Nick Merrill, a spokesman, said in a statement late on Tuesday that Clinton spent time at home "catching up on reading briefings, making calls," and watching Barack Obama's speech in Philadelphia, in which the US president offered unstinting praise of his former secretary of state.

Obama hit out at "unfair" criticism of Clinton and insisted that she had "been subjected to more scrutiny and ... more unfair criticism than anybody out here".

"Donald Trump says stuff every day that used to be considered as disqualifying for being president," Obama said.

"And yet because he says it over and over and over again, the press just gives up."

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Source: News Agencies