Elon Musk secretly funded a conservative political group in recent years, according to four people familiar with his donations, illustrating quiet financial support for right-wing causes even before the billionaire entrepreneur in July endorsed former President Donald Trump's bid for re-election.

Two of the people familiar with the donations told Reuters that Musk's contributions to the organization, Building America's Future, had started by 2022.

One of those people and a third source said the donations amounted to millions of dollars, significantly boosting a group whose advertisements and social media campaigns have criticised the Biden administration and progressive political platforms of the sort that Musk himself has increasingly denounced.

Reuters was unable to determine a precise amount and timeline for the contributions or identify documentation linking the organization's finances to Musk.

Earlier on Wednesday, The Wall Street Journal reported that Musk had financed other pro-Republican groups.

Musk didn't respond to emails seeking comment.

A spokesperson for Building America's Future didn't respond, either.

The magnate behind ventures including carmaker Tesla, space contractor SpaceX and the social media platform X, Musk for many years was careful to avoid suggestions that he favoured either major U.S. political party.

As recently as March, months before he publicly backed Trump and announced plans to finance a political action committee to work against Democrats, he wrote on social media: "Just to be super clear, I am not donating money to either candidate for US President."

Donations to Building America's Future, however, would show he was already using his vast resources to fund right-wing causes. As a non-profit 501(c)(4) group, the organization isn't required by federal law to disclose its financial backers.

Although such groups aren't allowed to finance candidates' political campaigns, they can espouse political causes.

As such, they are commonly referred to as "dark money" groups – used by political operatives, Democrats and Republicans alike, to hide the financial origins of influence campaigns.

In Musk's case, the four people familiar with his donations told Reuters, the contributions to Building America's Future remained closely guarded. Three of the people were briefed on the donations by executives at or linked to the organization and the fourth was consulted on the matter by a Musk aide.

All spoke with Reuters on the condition that they not be identified by name.

"STOP CACKLING KAMALA"

Revenues at Building America's Future, according to government data, climbed from some $11 million in 2021 to about $53 million in 2022, the year two of the people said Musk had already started his donations.

The figures are the most recent available from the Internal Revenue Service, which requires tax-exempt organizations to disclose their revenues.

It's unclear whether Musk still funds the organization or how much in total he may have donated.

Over the last two years, Building America's Future has attacked the Biden administration on a host of topics, including illegal immigration, an issue that Musk frequently comments on.

One recent anti-immigration video posted online by the group claims that Vice President Kamala Harris, Trump's opponent in the November election, "led the invasion" of migrants across the Mexican border and has always "put illegals first."

Building America's Future also recently launched a $10 million advertising campaign meant to undermine Black support for Harris, according to an August report by NBC News. The campaign criticizes the White House's effort to ban menthol cigarettes. Research shows the cigarettes, long marketed to African-Americans, are even more dangerous to smokers' health than regular tobacco.

"Instead of focusing on important issues," one videosays, "Biden's priority is banning menthol cigarettes," trying to tell adults "what they can and cannot do."

Musk's political leanings have moved rightward in recent years.

Although he has said he has voted for Democratic presidential candidates including Biden and Hillary Clinton, Musk became an outspoken critic of the current administration, claiming the White House gave a "very cold shoulder" to Tesla and SpaceX. A White House spokesperson declined to comment.

Musk, ranked by Forbes as the world's richest individual, has also become a fierce critic of identity politics. He has used his frequent posts on X to propagate demonstrably false conspiracy theories about Jewish people, immigrants and looming "civil war" in Britain.

After Musk's recent embrace of Trump, the former president said if elected he would put Musk in charge of a government efficiency commission.

America PAC, a political action committee Musk recently said he is financing, as of this week has spent $77 million on a get-out-the-vote campaign to encourage infrequent voters to support Trump, according to federal electoral filings. Musk's exact financial contribution to America PAC is unclear.

In addition to their Musk link, America PAC and Building America's Future overlap in terms of personnel, according to electoral filings and people familiar with the operations of both groups. One of those people is Generra Peck, a former campaign manager for Florida Governor Ron DeSantis and now one of the key leaders at America PAC. Peck didn't respond to a request for comment.

Like Building America's Future, America PAC's messaging has featured aggressive personal criticisms of Harris. Flyers distributed by the group in Arizona, reviewed by Reuters, feature photos of the vice president laughing and read: "stop cackling Kamala."