US attorney general William Barr has allowed federal prosecutors to probe alleged irregularities in the presidential election, prompting a top justice department official to quit.
The official, Richard Pilger, would have overseen such investigations.
Any such cases would normally be the remit of individual states, but Mr Barr said this was not a hard and fast rule.
Donald Trump refuses to accept Joe Biden’s projected victory, and has made unsubstantiated fraud claims.
The president’s campaign is seeking an emergency injunction in Pennsylvania to prevent Mr Biden’s victory being certified in the state.
The president-elect’s projected win there on Saturday took him over the threshold of 270 electoral college votes needed to secure victory nationwide.