A defiant Bernie Sanders said on Wednesday that he is moving ahead with his Democratic presidential campaign despite a bruising series of losses in primary elections on Tuesday.

Sanders told reporters in Burlington, Vermont, that he's not quitting even though his path to the presidential nomination narrowed considerably after decisive losses to Biden in Michigan, Missouri and Mississippi.

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"We are winning the generational debate," Sander said. "While Joe Biden continues to do well with older Americans ... our campaign continues to win vast majority of votes of younger people."

He cautioned Democratic leaders against ignoring the issues favoured by those young voters. "You cannot simply be satisfied by winning the votes of people who are older," he said.

Pressure has been growing on the independent Vermont senator to end his presidential bid and work to unify the party against President Donald Trump. But he said he's looking forward to a debate scheduled Sunday with Biden.

Sanders is scheduled to appear on NBC's "The Tonight Show Starring Jimmy Fallon" on Wednesday.

Four years ago, under similar pressure in a primary match-up against Hillary Clinton, Sanders fought on for months before ultimately backing Clinton in July.

Democrats have been pressuring Sanders to withdraw from the race and save them from months of a messy and expensive primary fight. But the 78-year-old democratic socialist has proven himself nothing if not willing to take on the political establishment against all odds.

The head of the most powerful super PAC in Democratic politics, Priorities USA, on Wednesday, pledged its allegiance to Biden.

"The math is now clear," tweeted Priorities Chairman Guy Cecil, "Joe Biden is going to be the Democratic nominee for president." He pledged his organisation would do "everything we can to help him defeat Donald Trump in November."

He added: "I hope others will join us in the fight."

But Sanders supporters have urged him to carry on his struggle, arguing that he is the only man who embodies the dramatic change they crave.

The Progressive Change Campaign Committee issued a statement even before the polls closed Tuesday calling on Sanders to not drop out - or at least not before Sunday's debate.

"No quick Biden coronation," said Maria Langholz, speaking for the group. "Win or lose tonight, Bernie should stay in the race until the March 15th debate at the earliest."