Elon Musk, one of the world's richest people, says he won't directly back Joe Biden or Donald Trump in the US election -- but one look at his X feed makes clear his heart is with the Republican.

Trump, meanwhile, needs cash -- burdened with millions of dollars in legal troubles, he is also gearing up to fight what will be the most expensive election in US history.

So could Musk ride to Trump's rescue?

In a recent interview with former CNN anchor Don Lemon, the owner of X, formerly Twitter, said he was "leaning away" from Biden, but maintained that it was "unlikely" he would contribute to any campaign.

If he chose to back a candidate, "I will provide a very detailed explanation of why I am endorsing one or the other."

For close Musk watchers, however, this seemed to edge away from a firm pledge days earlier to stay above the fray.

It came as Trump is picking up the phone to call multi-billionaires, hoping they will open their checkbooks and come to his rescue, according to The Washington Post.

Cold-calling those richer than him is not something Trump enjoys, but the former president is trailing Biden badly in fundraising and the campaign has still eight months to go.

- Billionaire breakfast -

Two of the world's most public, and controversial, figures, Trump and Musk have not always seen eye to eye.

Trump, for one, is not a believer in electric cars such as those that Musk, who is also chief at Tesla, has championed -- he has ridiculed them for being inconvenient and dependent on government subsidies.

During his 2016-2020 presidency, Musk kept his distance -- though that was before his public tilt to the far right.

Musk also flirted with Trump alternatives in the Republican primary, expressing support for Florida governor Ron DeSantis and entrepeneur Vivek Ramaswamy before they flamed out.

Observers agree that Trump has remained quiet on Musk's transgressions in hope for his money.

The Washington Post revealed that Musk attended a breakfast for billionaires in Florida last month where the former president pitched a group of donors.

That provoked swift speculation -- but Musk said "it was just breakfast," and that nothing more came out of it.

- 'Dark money' -

If Musk wanted to help Trump more directly, he could hand over limitless cash anonymously, according US election laws.

Called "dark money" by its critics, Musk could bankroll a candidate incognito via specially-created nonprofits called Super PACS.

"These groups can run ads that literally say, 'vote for Trump'" without any required disclosure on the source of the money behind them, said Shanna Ports, senior legal counsel at the Campaign Legal Center.

Another avenue is to contribute after the fact: Musk could help erase Trump's debts after the election, and thereby avoid riling the current White House occupant.

A rumor circulated in recent weeks that Trump was also pitching Musk to buy his social media platform Truth Social, and combine it with X.

Truth Social's parent company went public last week, with shares initially soaring on Wall Street. That meant Trump, who has nearly 80 million shares in the venture, has holdings now estimated in the billions, meaning he may be less desperate for cash.

But that wealth is on paper only, and financial experts warn that the mega-valuation could vanish if Trump ever tries to sell his shares.

- 'Combustible' -

Money aside, Musk's political inclinations give Trump a clear boost.

His presence on social media is often rife with unfounded conspiracy theories, disinformation and right-wing talking points that align with Trump, especially on immigration.

They also take place on a platform he owns -- where he has free rein to promote (or demote) certain content.

The US Federal Election Committee has received complaints in the past when platforms were perceived as biased, though it never acted on them.

"Those combined create a combustible moment for us where the experience on Twitter (X) is in the hands of a haphazard erratic billionaire who gets bored and uses it to promote his own views," said Nora Benavidez, senior counsel at the watchdog Free Press.

Donations or not, "Musk's influence is incredibly powerful," she added.