US president Donald Trump said he would pay every American adult a $5,000 "Trump dividend" if Republicans retain control of Congress in November's midterm election.

Trump made the proposal at his Republican Party's first-ever midterm convention, but it was not immediately clear how the payments would work or whether they would be legal.

With approximately 270 million US adults, the idea would cost around $1.35 trillion.

During a primetime address that lasted an hour and 45 minutes, Trump boasted of completing "the greatest two years in the history of the presidency," putting himself squarely at the centre of the election despite his deepening unpopularity.

But it was not clear whether his campaign-style speech would appeal to the critical swing voters who opinion polls show have drifted away from Trump's party, amid growing dissatisfaction with the cost of living and the war on Iran.