Washington DC - Elon Musk, billionaire Donald Trump supporter and America PAC founder, is putting up $1 million every day as a bonus to signatories of a petition encouraging people to vote in key swing states.

The petition supports "free speech and the right to bear arms" and pledges that every day from Saturday every person who signs the petition will be selected to have a chance of earning the prize.

Speaking at a rally in Harrisburg, Pennsylvania, Musk said his aim was to get between 1 million and 2 million voters in swing states to sign "because I think this sends a crucial message to our elected politicians."

He handed over the first check to a man from the state.

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Musk's aim appears to get Republican supporters in particular to register to vote.

Observers suggested that the stunt could contravene a law prohibiting paying voters to register.

According to the Federal Election Commission (FEC), Musk has donated around $75 million over the past three months to the America PAC, the political action committee he set up to support Donald Trump's campaign.

Musk has also used his public profile to attack Democratic Party candidate Kamala Harris.

Trump has indicated that he would appoint Musk to head a government efficiency commission with the aim of curbing US federal expenditure if he wins the presidency.