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!”.

 Joe Biden announced Thursday he is running for the White House, positioning the veteran Democrat as a frontrunner among the many candidates seeking to challenge Donald Trump in 2020.

The party’s 76-year-old senior statesman became the most experienced and recognized Democrat in the race, a popular former vice president dominating early polls following months — even years — of campaign planning.

In a tweet accompanied by a three-and-a-half minute video, Biden said he could not stand idly by as US President Donald Trump “fundamentally altered the character of this nation”.

“The core values of this nation… our standing in the world… our very democracy… everything that has made America — America — is at stake,” he wrote in the post.

“That’s why today I’m announcing my candidacy for President of the United States.”

Biden, whose working-class appeal remains intact despite nearly half a century in Washington politics, is seen as a comforting, known quantity for American voters who will be vetting some 20 Democrats now officially in the presidential field.

But recent controversy over his tactile style, particularly with women, could dampen a rollout that he envisioned as the final main entry to the Democratic primary battle.

Even before his official announcement, Biden, who served eight years as Barack Obama’s vice president, led most surveys of Democratic voters.

The RealClearPolitics poll aggregate puts him a favourite with 29.3 per cent support, followed by independent Senator Bernie Sanders at 23 per cent.