Video report by ITV News correspondent Juliet Bremner
Vermont Senator Bernie Sanders has said he is running for president in 2020.
The 77-year-old self-described democratic socialist unsuccessfully challenged Hillary Clinton in the Democratic presidential primary in 2016, and was widely credited for reshaping the party's politics and moving it to the left.
“Our campaign is not only about defeating Donald Trump,” 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.”
During his last presidential bid, Mr Sanders earned a loyal following with his passionate defence of liberal proposals, including free college tuition and single-payer healthcare.
But he will face off against several other Democratic candidates who also want to appeal to the party’s base.
However, Mr Sanders’s name recognition, fundraising prowess and passion for liberal policies make him a top-tier 2020 presidential contender.
If successful in becoming the Democratic presidential candidate, Mr Sanders would likely challenge incumbent Donald Trump.
The question now for Mr Sanders is whether he can 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, a far-cry from 2016, when he was Ms Clinton’s lone progressive adversary.
Despite losing the Democratic nomination three years ago, Mr Sanders still manage to win 13 million votes and dozens of primaries and caucuses.
Mr Sanders' brother, Larry, who is a Green Party spokesman living in England, told ITV News: "He's not crazy keen about being elected, he is crazy keen about turning around the horrible political situation. Partly, of course, it's Trump, who he - and I agree - is a liar and a racist and a disaster for all concerned.
"It's not only Trump, we've had 40 years in which we've shifted the money and the wealth, the income from the bulk to the very rich and it's had an impact, it got Trump elected and I think it's why we Brexit happened here.
"Bernard thinks none of the other candidates are as concerned and as committed and as clear about what needs to be done as he is."
Continuing to champion his brother the elder Sanders - who has lived in the UK since 1969 - said that what his brother stood for meant he was destined to become "a world significant person".
Larry Sanders previously described his brother as "hard hitting pro-people candidate" whose tough childhood, growing up without much money in Brooklyn, New York, means he can connect with the everyday American.
"Bernard is genuinely angry, more than angry at what he sees being done to people for no good reason , just so some people who already have a billion can have two.
"And it's not just the United States, it applies to Britain and other countries.
"We've had 30 or 40 years in which wealth and power and money and income have been redistributed from the bulk of the population to the very richest people."
Of his brother’s decision to return as a Democrat contender, the 83-year-old said: “He did ask me, I think for the first time in his life, what I thought about this a couple of months ago.
“I said, ‘as your brother I think you shouldn’t do it because you could have a marvelous life, but as a world citizen you have to’. That’s my opinion.”