Elon Musk greets Donald Trump onstage at the former president's campaign rally Oct. 5 in Pennsylvania. JIM WATSON/AFP via Getty Images

Two Democratic senators have urged the Department of Justice to investigate billionaire Elon Musk's $1 million lottery aimed at signing up pro-Donald Trump voters in seven swing states.

The CEO of Tesla and SpaceX, who announced the election sweepstakes in Pennsylvania a week ago, is paying out $1 million a day to some lucky registered voter in swing states who signs a petition supporting the right to free speech and the right to bear arms. The scheme provides a major incentive to register to vote, and also gathers information about the signer.

Sens. Peter Welch of Vermont and Richard Blumenthal of Connecticut, members of the Senate Judiciary Committee, sent a letter to Attorney General Merrick Garland on Friday saying Musk's giveaway violates federal campaign finance law against paying someone to vote.

"There is no place for vote-buying in our democracy. As the Department has recognized, voting should never 'degenerate into a spending contest, with the victor being the candidate who can pay the most voters,'" the lawmakers wrote in the letter.

CNN reported Wednesday that the Department of Justice warned in a letter to Musk's America PAC that his scheme may violate federal laws against paying people to register to vote. But the Tech mogul has continued to make the payments.

The senators complained that "permitting this scheme to proceed without consequences makes a mockery of democracy and the law."

They added: "We urge you to investigate whether Elon Musk's cash prizes are prohibited payments for voter registration and take appropriate enforcement action, including prosecution, if his actions prove to be illegal."