When Donald Trump takes to the stage at New York's Madison Square Garden today, he's expected to have the world's richest person on hand to help him rally the crowd.
Elon Musk has emerged as a major player in the former president's election efforts, unveiling attention-grabbing initiatives such as giving away $US1 million ($1.5 million) every day until November 5.
To be eligible, you would need to be a registered voter in one of the seven swing states expected to decide the election.
You'd also have to sign a petition supporting the first and second amendments of the US constitution (the ones relating to the right to free speech and to bear arms).
Election experts were quick to raise concerns that the cash bonanza could breach laws preventing people from being paid to register to vote or to cast a ballot. But the billionaire hasn't been deterred.
So how did Musk become such a central figure at this election, and what's in it for him?
A volatile relationship
The former president and Space X owner haven't always had such a cosy relationship.
At a campaign rally in 2022, Trump accused the billionaire of lying about who he'd voted for in previous presidential elections, describing him as a "bullshit artist".
Musk responded by suggesting Trump should "hang up his hat and sail into the sunset".
[EM twitter]Musk reinstated Trump's Twitter account later that year, after buying the social media company now known as X.
But he then used the platform to launch the failed presidential bid of Florida Governor Ron DeSantis, who was challenging Trump for the Republican nomination.
And when Musk declared last year he would not vote for Joe Biden despite having supported past Democratic candidates, he added: "I'm not saying I'd vote for Trump."
The Wall Street Journal traces Musk's move away from the Democratic Party to Biden's snubbing of the Tesla co-founder at electric vehicles events in 2021.
Musk, now 53, had previously described himself as politically "moderate", but publicly criticised COVID-19 lockdowns and moved two of his companies out of California following the passage of a law relating to gender identity in schools.
He's since used X to decry what he calls the "woke mind virus" and to amplify right-wing conspiracy theories — including those spread by Trump.
By May this year, he had launched a political action committee (PAC) to support Trump's re-election bid.
And in July, when the former president survived an assassination attempt at a rally in Pennsylvania, Musk formally endorsed him.
[EM endorses Trump]A legally problematic cash splash
Musk has since become a vocal advocate for Trump, jumping on stage alongside the former president at a recent rally and drumming up support for him on X.
But his biggest contribution comes in the form of cash.
"Usually the high-visibility people who get involved in political campaigns are entertainers and sports figures, but business people tend to give the money quietly," said Erik Gordon, a professor at the University of Michigan's Ross School of Business.
"They're not looking to attract a lot of controversy.
"But [Musk] is not one to shy away from any controversy. In fact, he seems to be very comfortable being right in the middle of controversy."
Musk has reportedly poured at least $U118 million ($178 million) into the America PAC, which is legally allowed to raise and spend money to support a candidate but can't directly coordinate with a campaign.
Campaign finance lawyer Brendan Fischer believes the PAC's original offer to pay $US47 to people who referred others to sign the petition was legal because it wasn't directly incentivising someone to register to vote.
But he argues the $1 million a day giveaway goes too far.
"At first glance, this might appear to be a very technical violation," he said.
"But billionaires and corporations and dark money groups are already spending hundreds of millions of dollars trying to influence our vote.
"If direct payments to voters to register and to vote is allowed to become the norm, then a new avenue for wealthy special interest money is going to open in our elections."
The Department of Justice (DOJ) has reportedly written to the America PAC, warning the initiative could violate federal law.
The daily announcement of winners was briefly paused, however the giveaway has since resumed.
"I would not expect that the DOJ would initiate a criminal action in the two weeks before the election," Mr Fischer said.
"But at this point, Elon Musk and America PAC are on notice that the Justice Department views what they are doing as unlawful. And the question now is whether they're going to change course."
What's in it for Musk?
Musk is a hugely influential figure across the tech, auto and space industries.
He counts the US government as a major client, with Space X holding multi-billion-dollar contracts with NASA and the Defense Department to launch their rockets and satellites.
Trump has promised that if he's elected he will set up a "government efficiency commission" tasked with carrying out audits across the federal government and recommending "drastic reforms".
He says the idea was suggested by Musk and that the billionaire has agreed to lead it — an appointment that would likely raise questions about potential conflicts of interest.
Professor Gordon said he expected Musk would use such a role to try to force changes within the federal regulatory bodies he's previously sparred with.
"I think he will try to influence regulation policy about space," he said.
"I think space is probably even more important to him than the cars. But he thinks that Tesla has suffered greatly at the hands of red tape.
"He has tangled with the Securities and Exchange Commission. Musk has done a bunch of arm wrestling with them. I think he'll try to streamline their processes."
Professor Gordon believes Musk's overriding motivation is to help elect an administration that would take his ideas seriously.
"He wants to have the freedom to do what he wants to do. He wants to push autonomous vehicle technology forward, he wants to push space technology forward to Mars," he said.
"And he sees government as being an impediment to progress."
Musk has drawn the ire of some Democrats, including vice-presidential candidate Tim Walz.
"Elon's on that stage, jumping around, skipping like a dipshit on these things," he told supporters at a rally in Wisconsin. "You know it," he said.
Biden also criticised the billionaire, seizing on allegations that Musk — who was born in South Africa — worked illegally in the US early on in his career.
"He was violating the law. He's talking about all these 'illegals' coming our way," Biden said in reference to Musk's embrace of Trump's immigration stance.
But John Fetterman, a Democratic senator for the must-win state of Pennsylvania, warned his party it would mock Musk at its peril.
"Musk is incredibly popular and he has an appeal to a demographic that Democrats have struggled with," he told The Hill.
"And to some people, they see him as … [Marvel character] Tony Stark. He's the world's richest man and he's undeniably a brilliant guy."
Musk is hardly the only billionaire involved in this election.
Trump has several other so-called "mega-donors", while Microsoft founder Bill Gates is reported to have privately donated $US50 million to an organisation supporting Ms Harris.
In what could be the most expensive election in US history, their influence will be consequential.
