After months of deliberation, veteran Democratic Senator Bernie Sanders has announced he will challenge Donald Trump for the US presidency, promising “an unprecedented and historic grassroots campaign”.

Senator Sanders, whose radical 2016 presidential campaign reshaped the tone of Democratic politics, told Vermont Public Radio (VPR) late Tuesday night (Australian time) he wanted to end the Trump presidency.

“I think the current occupant of the White House is an embarrassment to our country,” the 77-year-old self-described democratic socialist said.

“I think he is a pathological liar … I also think he is a racist, a sexist, a homophobe, a xenophobe, somebody who is gaining cheap political points by trying to pick on minorities, often undocumented immigrants,” he added.

Mr Sanders stunned the Democratic establishment in 2016 with his spirited challenge to Hillary Clinton.

While Ms Clinton ultimately became the party’s nominee, his campaign helped lay the groundwork for the leftward lurch that has dominated Democratic politics in the Trump era.

“I am asking you to join me today as part of an unprecedented and historic grassroots campaign that will begin with at least a million people from across the country,” Mr Sanders said in an email to supporters.

“Our campaign is about transforming our country and creating a government based on the principles of economic, social, racial and environmental justice,” he said.

Before he can pose any challenge to Mr Trump, the Vermont Senator must stand out in a crowded field of Democratic presidential candidates who also embrace many of his policy ideas and are newer to the national political stage.

Still, there is no question that Senator Sanders will be a formidable contender for the Democratic nomination.

He won more than 13 million votes in 2016 and dozens of primaries and caucuses. He opens his campaign with a nationwide organisation and a proven small-dollar fundraising effort.

In 2016, Senator Sanders notably struggled to garner support from black voters, an issue that could become particularly pervasive during a primary race that could include several non-white candidates.

The presidential hopeful attracted criticism when he addressed the diversity of candidacy rivals while speaking on VPR.

“We have got to look at candidates, not by the colour of their skin, not by their sexual orientation or their gender and not by their age,” he said.

“I think we have got to try to move us toward a nondiscriminatory society which looks at people based on their abilities, based on what they stand for.”

Senator Sanders also faces different pressures in the MeToo era. Some of his male staffers and supporters in 2016 were described as “Bernie bros” for their treatment of women.

While the President is yet to comment on Senator Sander’s candidacy, the Trump re-election campaign issued a statement that reflected Mr Trump’s strategy of labelling his Democratic opponents as “socialists”.

Kayleigh McEnany, press secretary for the Trump campaign, said Senator Sanders had already won the Democratic debate because “every candidate is embracing his brand of socialism”.

Ms McEnany’s statement also criticised Senator Sanders for supporting higher taxes on wealthy Americans to help finance “Medicare for all”.

-with agencies