Pennsylvania's Democratic governor Josh Shapiro is among those calling for an investigation of billionaire Elon Musk, for his promise at a weekend pro-Trump rally to give away $1 million US each day until election day in the U.S.

Musk on Saturday gave a $1-million US check to an attendee at the rally in Harrisburg, Pa., which was hosted by America PAC, a political action group the Tesla CEO set up to back Republican Donald Trump in the Nov. 5 presidential election.

Shapiro said on NBC's Meet the Press that Musk's  plan to give money to registered voters in Pennsylvania is "deeply concerning" and is "something that law enforcement could take a look at."

The money is the latest example of Musk using his extraordinary wealth to influence the tightly contested presidential race between Trump and his Democratic rival, U.S. Vice-President Kamala Harris. Pennsylvania is a must-win state for both Trump and Harris.

Musk's America PAC is playing a major role in helping mobilize and register voters in battleground states that could decide the election, but there are signs it is having trouble meeting its goals, Reuters was first to report on Friday.

The lucky recipient of Saturday's check was a man named John Dreher, event staff said.

"By the way, John had no idea. So anyway, you're welcome," Musk said as he handed Dreher the check.

On Sunday, Musk handed a second $1-million US check to a woman at an event in Pittsburgh, according to a post by America PAC on X.

Musk, ranked by Forbes as the world's richest person, so far has supplied at least $75 million US to America PAC, according to federal disclosures, making the group a crucial part of Trump's bid to regain the White House.

The Harris campaign hasn't officially commented on the giveaways, though in a fundraising email to encourage supporters to donate early Monday, said "[Trump's] buddy Elon Musk is spending huge sums of money on his own ads hammering the vice-president."

Only swing state voters eligible

Musk, born in South Africa and for a time a Canadian resident while in university, is promising to give $1 million US each day to someone who signs his online petition, which 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."

To be eligible for the $1 million US, petition signers must be a registered voter and live in one of the seven swing states: Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin, according to the America PAC website.

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The petition also offers $100 US to each registered Pennsylvania voter who signs and $100 US for referring a registered Pennsylvania voter to sign.

The legality of the giveaways is sure to come under scrutiny in the coming days.

It is a federal crime to pay people with the intention of inducing or rewarding them to cast a vote or to get registered, an offence punishable by prison time. The prohibition covers not only monetary expenditures, but also anything of monetary value like liquor or lottery chances, a Justice Department election-crimes manual says.

The Harrisburg event was the third in as many days in Pennsylvania, where Musk is painting November's election in stark terms and encouraging supporters to vote early for the Trump ticket with vice-presidential nominee, Sen. J.D. Vance of Ohio. Attendees of 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.

Vance suggested last week that "big technology companies" helped sway the 2020 election when a New York Post story critical of Joe Biden was temporarily banned from appearing on Facebook and Twitter, renamed X by Musk. Vance has not commented negatively on the X owner's partisan efforts this cycle.

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After denying he was a Republican, Musk has increasingly supported the party's causes and this year became an outspoken Trump supporter.

Trump in turn has said if elected he would appoint Musk to head a government efficiency commission, which would seemingly be rife with apparent conflicts given that Musk's companies, which also include SpaceX, Tesla and Starlink, are either subject to regulation or have contracts with government entities.

Trump, who was campaigning Sunday in Pennsylvania, was asked about Musk's giveaway, and said, "I haven't followed that."