• Video report by ITV News Washington Correspondent Robert Moore

The process of picking the Democratic candidate to take on US President Donald Trump in this autumn's election may only just have begun.

But the man previously seen as a political outsider - Bernie Sanders - is now seen as the one to beat.

So far only three of the more than 50 states and territories have voted.

Though a strong showing from the 78-year-old Senator in South Carolina on Saturday and a few days later on Super Tuesday - could put him beyond the rest of the divided field of democratic hopefuls.

It is something of a riddle. Why does this 78-year-old senator from Vermont have such a remarkable appeal to a younger generation of Americans?Bernie Sanders is hoping it takes him to the Democratic nomination and all the way to the White House.

His message helps. Not just populist talk against what he calls the new oligarchy but also his praise of young voters as this country's agents of change.

Crowds show their support for Bernie Sanders. Credit: AP

"This is generation that is anti-racist, anti-sexist, anti-homophobe - in fact they are anti everything Trump is for," he told crowds of supporters in Virginia.

If Bernie Sanders does not win the nomination then one critical question emerges: will his core supporters vote for a centrist or establishment candidate? In other words, is it Bernie or bust?

Many at a Sanders rally told ITV News Washington Correspondent Robert Moore they would be reluctant to vote for any other democratic nominee.

"Passion is where you get voters. People don't come to vote and stand out in the cold unless they're passionate. Fear doesn't drive people, passion does," said one Sanders supporter.

This could end up being an election against two populists. Vastly different, but leaving many Americans with nowhere to turn.