US President Donald Trump has pledged to give $5,000 to every American adult if the Republican Party retains control of both the House of Representatives and the Senate in the November midterm elections.

Trump offered no details on how the plan would work or where the funding would come from.

The pledge, if implemented, would cost the US government an estimated $1.3 trillion.

Federal law prohibits payments made in exchange for votes, raising immediate legal questions about the proposal.

Democrats dismissed the announcement as an "empty promise," and others have questioned whether such a pledge is even lawful.