Donald Trump is rebuffing demands that he provide evidence for the claim that he has given “tens of millions of dollars” to charitable organisations as doubts about it grow.

Boasts about his allegedly generous levels of charitable giving have become a staple of his campaign. On Monday, his running mate, Governor Mike Pence, called him “a man who’s given away tens of millions of dollars to charitable causes throughout his business life”.

But calls both from media outlets and from Hillary Clinton for proof of the boasts are meeting resistance from Mr Trump. His campaign manager, Kellyanne Conway, said merely, “I doubt it”, when asked on Tuesday by CNN if he would release a summary of his charitable giving.

Their stonewalling is only likely to increase speculation, however, that Mr Trump is being cavalier with the facts when it comes to his self-declared commitment to charitable causes such as care for veterans and support for the police. Additionally, there is still nothing to indicate that Mr Trump is preparing to buckle to pressure that he release his personal tax returns.

His campaign this week issued a statement standing by the “tens of millions” number and emphasising the works of his charitable entity, the Trump Foundation. It said that the candidate gave “tens of millions of dollars to charities both through his Foundation and otherwise”.

It concluded: “Mr. Trump serves the Foundation without compensation and additionally makes regular personal contributions to charities and causes of his choosing outside of the foundation. All in all, the Foundation supports many worthy causes, and the Foundation distributes its funds to get the money into the hands of those in need as soon as possible”.

However, investigative reporting by both CNN and The Washington Post has found that Mr Trump has not in fact given a cent of his own money to the Foundation that bears his name since 2008 and that it is funded instead by donations from third parties.

That is an unusual arrangement in the world of philanthropy. “It’s uncommon for a private foundation with somebody's name on it to not be receiving contributions from that individual,“ said Richard Marker, a philanthropy professor at the University of Pennsylvania.

Mr Trump first encountered media scepticism in January when he missed a debate in Iowa and instead held a televised benefit for veterans and then claimed it had raised $6 million for veterans groups. After repeated media requests for details, he eventually confirmed the figure in May and said that $1 million had come from his own pocket.

That was true, though it was widely portrayed as the only way Mr Trump could push the tally up to the $6 million level he had previously described. He also recently gave $100,000 of his own money to a church in Louisiana to help the clean-up after the recent flooding in the state.

But a review by The Washington Post found that 326 charities with some kind of connection with Mr Trump had received only one single gift in the amount of $10,000 that had actually come from his pocket between 2008 and May this year.

In its article on Mr Trump’s charitable giving record, the Post also highlighted a political donation that the Foundation had given to Florida Attorney General Pam Bondi at a time when she was considering launching an investigation into the doings of Trump University. It also suggested that the Foundation had been used to purchase items for Mr Trump himself, including a $12,000 football helmet and a $20,000 painting of the Republican nominee.

“This article is not at all accurate,“ Hope Hicks said in her statement on Monday. "Mr. Trump continues to be unfairly maligned for his generosity and we will continue to correct the record.”

In her remarks on Tuesday, Ms Conway, elevated to campaign manager last month, suggested that the media was at fault for spending too much time asking questions about the issue.

“This is like badgering,” she said. “I don’t see it as journalism, I see it as badgering. We’ve had this conversation so many different times on so many different networks, yet we’re not having a conversation about what the middle class tax relief would mean for people’s wage stagnation.”

If Mr Trump is getting growing numbers of questions about his Foundation, then some Democrats will see it as just desserts after the barrage of criticism he has directed at Ms Clinton and her husband, Bill Clinton, for alleged conflicts of interest arising from their adminstration of the Clinton Foundation.