Billionaire Elon Musk has pledged to give away $US1 million ($1.49 million) a day to American voters who sign his petition backing the US constitution.

Musk has already handed out two $1 million cheques to attendees at a rally in Pennsylvania he organised, aimed at boosting support for Republican presidential nominee Donald Trump.

The first winner was a man named John Dreher, according to event staff.

“By the way, John had no idea. So anyway, you’re welcome,” the Tesla founder said as he handed Dreher the cheque.

Musk handed the second $1 million cheque to an overjoyed Kristine Fishell.

“Thank you so much. I truly believe everything you’re doing. You don’t need to be doing it,” she said.

“Your wealth and responsibility you are using to save speech, and we just all appreciate it. Thank you so much.”

The money, which Musk said he would hand out every day until the November 5 presidential election, is the latest example of him using his extraordinary wealth to influence the tightly-contested race between Trump and his Democratic rival Kamala Harris.

Kristine Fishell received Musk’s second seven-figure cheque. Photo: Getty

Musk started America PAC, a political action organisation in support of Trump’s presidential campaign.

It is helping mobilise and register voters in battleground states, but there are signs it is having trouble meeting its goals.

The Harrisburg event on Saturday (local time) was the third in as many days in Pennsylvania.

Musk has painted November’s election in stark terms and encouraged supporters to vote early and get others to do the same.

He said that if Harris won on November 5, it would be “the last election”, suggesting the US would no longer exist.

Musk also said the two assassination attempts against Trump proved he was ruffling feathers and upending the status quo in ways Harris wouldn’t.

He said that was why no one was trying to kill Harris.

“Assassinating a puppet is worthless,” Musk said, reiterating an argument he has made in a social media post.

The petition Musk wants people to sign reads: “The First and Second Amendments guarantee freedom of speech and the right to bear arms. By signing below, I am pledging my support for the First and Second Amendments.”

Attendees at Saturday’s event had to sign the petition, which allows America PAC to garner contact details for more potential voters that it can work to get to the polls for Trump.

But the $1 million pledge has raised concern among US election law experts and Pennsylvania Governor Josh Shapiro.

He was previously the state’s attorney-general and is also involved in Harris’s election campaign.

“I think it’s something that law enforcement should take a look at,” Shapiro said on NBC’s Meet the Press on Sunday.

“I’m not the attorney-general any more of Pennsylvania, I’m the governor, but it does raise serious questions.”

UCLA election law professor Richard Hasen told news website NPR that Musk’s offer didn’t appear illegal at first. But a closer look raises red flags.

“If you look at the conditions, you must be a registered voter,” Hasen said.

“This is essentially a lottery that’s open only to people who register to vote. So it’s either an incentive for someone to vote or it’s a reward. And either way, it violates federal law.”

Musk, ranked by Forbes as the world’s richest person, has so far given at least $US75 million to America PAC, according to federal disclosures. That level of funding makes the group a crucial part of Trump’s bid to regain the White House.

The entrepreneur behind carmaker Tesla and rocket and satellite venture SpaceX has increasingly supported Republican causes and this year became an outspoken Trump supporter.

Trump, in turn, has said if elected he will appoint Musk to head a government efficiency commission.

-with AAP