Reuters reported that Mr Mellon, an heir of the Pittsburgh-based Mellon banking family, gave $50m. Another $10m came from billionaires Liz and Dick Uihlein. US media reported that Mr Mellon sent the donation the day after Trump was convicted on 34 charges of falsifying business records in his New York hush-money trial.

Mr Mellon has also been the biggest donor to independent presidential candidate Robert F Kennedy Jr, giving the pro-Kennedy Super-Pac American Values at least $20m. Super-Pacs are independent “political action committees” that can raise unlimited amounts of money to support an election candidate.

The Mellon donation has helped pro-Trump allies outspend US President Joe Biden’s allies on campaigning in recent weeks. Mr Mellon, who lives in Wyoming and is rarely photographed, is an amateur pilot who has invested in and led transport-related companies.

Forbes estimates that the Mellon family is worth around $14.1bn.

Meanwhile, Reuters reported that billionaire Mike Bloomberg has given nearly $20m to Biden-supporting Super-Pacs. Mr Bloomberg’s contribution included $19m to an independent pro-Biden group known as Future Forward or FF PAC. A further $900,000 was given to the Biden Victory Fund, which is an amalgamation of the Biden campaign and Democratic Party committees.

The Biden campaign did not immediately respond to a request for comment. On Thursday, the pro-Kennedy Super-Pac reported that it took in only around $280,000 last month, with none of it coming from Mr Mellon.