United States President Donald Trump says he would offer every adult American a five-thousand-dollar payout if Republicans win control of both chambers of Congress in the November midterm elections.

The proposal, which could cost the US government more than one-point-three trillion dollars, was announced during the Republican Party's first convention held in the middle of a presidential term.

Trump gave no details on how the payments would be structured or funded, saying only that the money would have to be spent within the United States.

Vice-President J.D. Vance has linked the proposal to revenue generated by the Trump administration through tariffs.

Democrats have dismissed the plan as an empty promise, while other critics have questioned whether such a payment would be legally permissible or financially feasible.

With nearly 270 million adults in the United States, a five-thousand-dollar payment to each adult would cost an estimated one-point-three trillion dollars.