US President Donald Trump cast doubt on Joe Biden's mental capacity for a successful primary campaign.

US President Donald Trump on Thursday cast doubt on Joe Biden's mental capacity for a successful primary campaign as the seasoned Democrat became the latest to join the 2020 race for the White House.

"Welcome to the race Sleepy Joe. I only hope you have the intelligence, long in doubt, to wage a successful primary campaign," Trump said on Twitter.

"It will be nasty - you will be dealing with people who truly have some very sick & demented ideas. But if you make it, I will see you at the Starting Gate!"