Elon Musk and his aides have taken control of the US Treasury Department's payments system -- which manages trillions of dollars of transactions each year -- sparking alarm among critics. The Treasury's closely guarded payments system handles the money flow of the US government,

In a letter to the Treasury secretary, the Democratic senator warned against meddling with a system that processes trillions of dollars of transactions.

Trump insists that anything Musk does must first be approved by the president.