The artist of a six-foot tall portrait of Donald Trump, which the presidential candidate bought for $20,000 using charitable money, said he hoped his artwork was being used for a good cause.

The Republican nominee spent thousands of dollars - from his own charitable foundation - to buy the portrait at a fundraiser in 2007.

Does the artist, Michael Israel, know what happened to the portrait?

"No clue," he told The Independent.

He said the painting might have been bought as an investment which could increase in value over time, or could be displayed, for example, in a hospital or autism centre.

"It’s fine if it [buying the painting] works with the charity’s mission," said Mr Israel.

"But if he [Mr Trump] just bought himself a present then that’s a different story."

Mr Israel said he painted the picture in less than 10 minutes at a fundraiser in Florida for a women and children's charity called HomeSafe.

Michael Israel beside Donald and Melania Trump in 2007 (Michael Israel Inc)

"I think it’s pretty amazing that it boils down to the painting being such a big focus - as much as I love my work, I think it's trivial in the big scheme of things," he said.

Mr Trump’s critics disagree.

In September, president Barack Obama highlighted differences between the Trump Foundation and Hillary Clinton’s family foundation.

"One candidate’s family foundation has saved countless lives around the world," he said.

"The other candidate’s foundation took money other people gave to his charity and then bought a six-foot-tall painting of himself."

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During the third presidential debate, Ms Clinton said: “I'd be happy to compare what we do with the Trump Foundation, which took money from other people and bought a six- foot portrait of Donald. I mean, who does that? It just was astonishing.”

Mr Trump spent another $10,000 on a portrait of himself in 2014, and it was later found hanging on the wall of a sports bar at his Doral golf resort.

He also bought a signed NFL helmet.

As reported by The Washington Post, the Republican nominee has not donated funds to his foundation since 2008, and has mainly acted as an intermediary between individuals and other charities.

The foundation, run by him and his children, work on the cause for half an hour per week, according to tax records.

New York attorney general Eric Schneiderman ordered Mr Trump at the start of October to stop soliciting cash immediately while it was under investigation for fundraising without being registered under state law.

"Melania [Trump] and I had a discussion about artwork and the charity, and I thought she was very elegant, very smart and educated, compared to some conversations I’ve had with people," said Mr Israel.