"Federal law is clear. He can't pay someone to vote or to register to vote. That is prohibited by 18 USC 597 and 52 USC 10307. Instead, Musk is paying people to sign a petition supporting the First and Second Amendments if the person is registered to vote in a swing state. He is also giving away $1 million a day until the election," Rahmani said.

The letter from federal election law enforcement officials comes amid Musk's growing stature as a Trump political supporter and donor.