Donald Trump has promised to give $US5000 (NZ$8500) to every American adult if Republicans retain control of Congress after the midterm elections.

US President Donald Trump has promised a $5,000 payment to every adult American if Republicans win control of both chambers of Congress in November’s midterm elections.

The US president's US$1.35 trillion pledge raises a number of questions, and he has not specified where the money would come from.

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