US Senator Bernie Sanders has promised to unleash 'a political revolution which is going to transform America'.

Early signs indicate Bernie Sanders will win the Democratic nomination and get the chance to run against Donald Trump for the US presidency in 2020.

Senator Sanders has raised US$18 million (A$25 million) since launching his 2020 presidential campaign, a substantial sum which, coupled with strong polling, puts him atop the pack of declared contenders for the Democratic nomination.

“We are building an unprecedented coalition to not only defeat (Donald) Trump but transform the country into one that works for all people,” the liberal US senator said on Twitter.

Since announcing his candidacy on February 19, Sanders raked in US$18.2 million from 525,000 individuals, who gave an average of $20, a sign of considerable small-donor support.

Most donors were under 39 years old, a “huge” asset to the candidate, his campaign director Faiz Shakir said on a press call, as the 77-year-old Sanders builds a grassroots revolution that has inspired younger voters.

The fundraising haul adds to millions of dollars that Sanders had raised previously through the network he built while running for president in 2016, when he lost the party’s nomination to Hillary Clinton.

He finished the first quarter of 2019 with US$28 million in the bank, according to Shakir.

“The state of the Bernie campaign is strong and getting stronger by the day,” Shakir said.

Two other candidates in the large herd of Democrats seeking to challenge Trump in 2020 have released their first quarter figures.

Senator Kamala Harris of California, who ranks third in polling behind former vice president Joe Biden — who has yet to enter the race — and Sanders, announced she raised US$12 million (A$16.9 million) from 218,000 individuals since her campaign launched on January 21.

Pete Buttigieg, the mayor of South Bend, Indiana, announced on Monday that his campaign raised US$7 million (A$9.9 million) in the opening months of 2019, an impressive haul given he was virtually unknown to most Americans just weeks ago.

Prominent candidates Beto O’Rourke and Senator Elizabeth Warren are among several 2020 hopefuls expected to release their first quarter figures in the coming days.

Candidates have until April 15 to file their fundraising reports with the Federal Election Commission.