In a Democratic presidential primary that has matured into a battle between Joe Biden and Bernie Sanders, people voting in the shadows of Marvel Stadium could play a key role in determining Donald Trump's election opponent.

Down Under Democrats Kristen Gordon (left) and and Laurie Staub at the polling booth in Docklands on Saturday. Simon Schluter.

This week, tens of thousands of US citizens living away from their homeland – including Sanders' own brother, who lives in England – voted to elect delegates who will select the Democrats' presidential nominee in July.

More than a thousand Americans living in Australia – including hundreds of Melburnians – voted on Saturday, with local organisers noticing a surge in turnout of voters energised to topple Trump.

The Democratic race tightened dramatically this week as Biden resurrected his bid with unexpected wins in key states like Texas, pushing him into favouritism ahead of fellow septuagenarian Sanders.

With Biden into a narrow lead, some pollsters and bookmakers are tipping a situation where neither candidate receives a majority of delegates to automatically win the primary.

Democratic presidential candidate and former vice-president Joe Biden takes a picture with the choir at the Brown Chapel African Methodist Episcopal Church in Selma, Alabama. Atlanta Journal-Constitution/AP

Overseas voters send 21 delegates to the party's convention, where a candidate requires 1990 delegates to win the nomination. Voters in Australia are expected to make up less than 5 per cent of the total overseas vote.

In a neck-and-neck race, each delegate is key – and no group of voters has a more outsized influence on the delegate pool than those who vote outside the US.

The Democrats' formula for apportioning delegates meant there was one delegate for every 2500 overseas votes in the 2016 primary. In comparison, about 11,000 votes equated to each delegate in the California.

Kent Getsinger, chair of the Australian chapter of Democrats Abroad, said the overseas vote in this tight primary could be "impactful". He compared the situation to the 2000 presidential election between Al Gore and George W. Bush, where a small number of overseas ballots proved crucial.

Sanders, accompanied by his wife Jane, speaks during a primary night election rally in Vermont earlier last week. AP

The Democratic Party is gripped by an internal conflict over its identity, with the divide evident at the polling station in Docklands.

Its younger generation, who are voting in droves for self-described "democratic socialist" Sanders, rails against globalisation and free trade and demands universal healthcare.

The moderate establishment wing, which is lining up behind Biden, advocates more incremental reforms.

If history is anything to go by, Sanders' left-wing agenda is a winner among Americans living Down Under. In 2016, the senator crushed Hilary Clinton 635-237 among voters living in Australia.

Laurie Staub is a Melbourne resident and a lifelong Democrat with decades of experience working on party campaigns, including Barack Obama's 2012 campaign where she met Biden and got to know his now-deceased son Beau.

She supported Sanders in 2016, but believes the likeability and pragmatism of Biden is what the party now needs.

"He's the most authentic, genuine, kind human being," said Ms Staub, who met the former vice-president for a third time when he visited Melbourne in 2016.

"The next four years in the United States needs someone to steady the ship.

"You can't dream all these wonderful ideas that Elizabeth Warren and Bernie Sanders have until you clean up the mess so you get back to the basic foundations, and then you build."

In the other camp is 25-year-old Stephen Underwood, who is scrutinising ballots at Docklands for the Sanders campaign.

He's young and passionate, and believes Sanders is the only person capable of making what he calls the "corrupt" economic system work in favour of working-class people.

"He has authenticity and absolute consistency across the years in pushing a progressive agenda, and he's willing to take on vested interests," Mr Underwood said.

"He doesn't talk shit."