The proposal, which would cost more than $1 trillion, could face legal challenges under federal election law

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.

President Donald Trump said in his keynote address that he promises $5,000 to every adult citizen if the Republicans win both the House and Senate in midterms.

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