WASHINGTON — US President Donald Trump mocked "little" billionaire and former New York mayor Michael Bloomberg on Friday, saying his potential opponent for the 2020 election would fail if he joined the race.

"He doesn't have the magic to do well... Little Michael will fail," Trump told reporters at the White House as he headed to the southern state of Georgia to rally support.

Bloomberg, 77, is expected to enter the race on Friday to become the Democratic nominee for US president, setting up a showdown with fellow septuagenarian Joe Biden as the leading centrist candidate.

He is expected to file paperwork in at least one state declaring himself a candidate, according to multiple outlets including The New York Times.

Bloomberg had said back in March he wouldn't run, but more recently has been toying with the idea of throwing his hat in the ring, according to an advisor.

He sent members of staff to Alabama to gather the signatures required to register for its primary ahead of the deadline Friday in anticipation of a bid, the reports said.

Bloomberg adviser Howard Wolfson said in a statement that the billionaire wanted to ensure that the job of defeating Trump was seen through in 2020.

"He's not going to do well, but I think he's going to hurt Biden actually," Trump added.

"There's nobody I'd rather run against than Little Michael." — AFP