BERNIE SANDERS HAS defied pressure to drop out of the Democratic primary contest for the sake of party unity, saying he remained in the fight against centrist frontrunner Joe Biden.

But in a brief, passionate speech, Sanders made clear that his main priority was to see the defeat of “dangerous” President Donald Trump.

And he acknowledged that many Democrats see him as less electable than Biden — perhaps preparing the ground for his exit.

Sanders had been holed up in his home state of Vermont overnight after suffering heavy defeats in primaries across the country Tuesday.

Speculation had been growing that he might respond to pleas from Democratic activists to step aside, giving Biden – his only rival – a chance to focus on campaigning against Trump.

But the fiery leftist broke his silence to say he was preparing for his first one-on-one televised debate Sunday “with my friend Joe Biden”.

While he remained defiant, Sanders did make a number of unusual statements, accepting both his losses during the primary race and perceptions that he would fare less well against Trump.

“I cannot tell you how many people our campaign has spoken to who say they agree with us but will vote for Joe because they believe he’s the best to beat Donald Trump,” he said.

“Needless to say, I strongly disagree with that assertion, but that’s what millions of Democrats and independents say.”

Sanders then listed questions he wanted Biden to answer on how to deal with far-reaching inequality and other structural issues that he believes can only be addressed by radical economic change or “revolution.”

“Joe, what are you going to do?” he asked, in what sounded less like an attack on Biden than an appeal for him to reach out to the party’s left-wing.

Biden’s trajectory to a November showdown with Trump began looking unstoppable after he scooped a series of primary wins on Tuesday, including in the industrial powerhouse of Michigan — a state that will be crucial in the general election.

Biden racked up wins in Mississippi, Missouri, Idaho and Michigan, building on his come-from-behind triumph a week earlier on Super Tuesday.

He now has about 860 delegate votes to Sanders’ 710, and looks set to reach the needed 1,991 delegates.

Addressing supporters in Philadelphia as his victory took shape, 77-year-old centrist Biden reached out to Sanders, thanking the leftist Vermont senator and his supporters for their “tireless energy and their passion”.

The former vice president under Barack Obama struck a unifying tone as he affirmed on national television that he and Sanders “share a common goal and together we will defeat Donald Trump”.

Another of the six states on offer, North Dakota, was called for Sanders early today. He also led the count by around 2,100 votes in Washington state, the other major prize of Tuesday’s primaries, with around 70%of the vote counted.

With Biden’s path to the Democratic nomination increasingly locked in, the overwhelming question is whether Sanders will swallow his pride and get his supporters behind the candidate – or risk party civil war.

“It will be vital for Sanders to mobilize his supporters behind the ticket,” tweeted Princeton University history professor Julian Zelizer.

“But Biden has to reach out to Sanders’s energized movement. It will require a two-way effort to unite the ticket.”

Many Democrats accuse Sanders and his devoted supporters of fatally damaging Hillary Clinton four years ago when she was struggling – ultimately unsuccessfully – against Trump. They warn a similar scenario could play out this time.

Reflecting the shift in power, Democratic super-PAC (political action committee) Priorities USA, which had been neutral in the race, came out in support of Biden after Tuesday’s results.

“The delegate math is now a straight line to Joe Biden’s nomination,” chairman Guy Cecil told US public radio network NPR. 

“So we’re going to do everything we can to help him in the effort looking forward to November.”