Joe Biden's trajectory to a November showdown with President Donald Trump looked unstoppable Wednesday after he scooped a series of devastating primary wins -- as the party waited anxiously to see if his rival Bernie Sanders would concede and rally behind the presumptive nominee.

Democratic presidential hopeful US Senator Bernie Sanders speaks at a “Bernie 2020” rally at the Stifel Theater in downtown St.Louis, Missouri on March 9,2020 Democratic presidential hopeful US Senator Bernie Sanders speaks at a “Bernie 2020” rally at the Stifel Theater in downtown St.Louis, Missouri on March 9,2020 Photo: AFP / Tim VIZER

Pressure was mounting on Sanders to end his campaign as Biden inflicted defeats in Mississippi, Missouri, Idaho and Michigan, carving a clear path to becoming the Democratic standard bearer in a potentially bruising matchup with Trump.

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."

Democratic presidential hopeful Senator Bernie Sanders speaks at a rally at the Stifel Theater in downtown St. Louis Democratic presidential hopeful Senator Bernie Sanders speaks at a rally at the Stifel Theater in downtown St. Louis Photo: AFP / Tim VIZER

The former vice president 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 Wednesday. He also led the count by around 2,100 votes in Washington state, the other major prize of Tuesday's primaries, with around 70 percent of the vote counted.

Democratic presidential candidate Joe Biden meets workers as he tours the Fiat Chrysler plant in Detroit Democratic presidential candidate Joe Biden meets workers as he tours the Fiat Chrysler plant in Detroit Photo: AFP / MANDEL NGAN

But Biden's win in Michigan, the Midwestern industrial state which could be a key battleground in November, was a major setback, prompting calls for Sanders to pull out.

People vote in the Michigan primary election at Chrysler Elementary School in Detroit People vote in the Michigan primary election at Chrysler Elementary School in Detroit Photo: AFP / JEFF KOWALSKY

The main question is whether Sanders, a self-declared democratic socialist, will fight to the bitter end, as he did four years ago against Clinton, or bow out early.

Democratic presidential candidate former vice president Joe Biden speaks during a campaign rally at Renaissance High School in Detroit, Michigan on March 9, 2020 Democratic presidential candidate former vice president Joe Biden speaks during a campaign rally at Renaissance High School in Detroit, Michigan on March 9, 2020 Photo: AFP / MANDEL NGAN

"Yes, 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."

Tom Moran holds a sign urging Democrats to unite at a rally for Bernie Sanders in Dearborn, Michigan Tom Moran holds a sign urging Democrats to unite at a rally for Bernie Sanders in Dearborn, Michigan Photo: AFP / JEFF KOWALSKY

Many Democrats accuse the firebrand Sanders and his devoted supporters of damaging Clinton just as she was struggling -- ultimately unsuccessfully -- against the Trump insurgency.

Sanders did not immediately address Tuesday's results.

People wait to put their ballots in the tabulation machine at Chrysler Elementary School in Detroit People wait to put their ballots in the tabulation machine at Chrysler Elementary School in Detroit Photo: AFP / JEFF KOWALSKY

His press secretary indicated he had no intention of pulling out, touting Sunday's next debate as the chance for America to "see Biden defend his ideas or lack thereof."

But Democratic superPAC Priorities USA, which had been neutral in the race, came out in support of Biden after Tuesday's results.

US presidential primaries: polls close in Michigan US presidential primaries: polls close in Michigan Photo: AFPTV / Agnes BUN

"What tonight has made clear is that 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."

Adding to nerves in a country on edge after three tumultuous years under Trump, fears of the coronavirus epidemic prompted both campaigns to cancel rallies planned in Cleveland, Ohio.

Organizers of the upcoming television debate between Sanders and Biden likewise said they would take the extraordinary step of not allowing the usual live audience.

Their caution was at odds with Trump, who has vowed to keep holding his typically raucous rallies despite the concerns about large gatherings.

Trump's campaign manager dismissed Tuesday's results, saying "it has never mattered who the Democratic nominee turns out to be."

Both are "running on a big government socialist agenda" and Trump "is on an unstoppable drive toward re-election," campaign manager Brad Parscale said.

Tuesday's six-state contest, coming a week after Biden's strong Super Tuesday performance, demonstrated that his once shambolic campaign is gathering strength and is now ready to face Trump.

Sanders had accused Biden of selling out to corporate interests and ignoring a passionate leftist base.

But the former vice president has insisted on steering a centrist line that he says can bring the divided country back together.

With stories of a hard-knock childhood frequently featuring in his speeches, Biden also wants to recapture the blue collar white voters that Trump successfully poached from the Democrats in 2016.

"Biden is putting together the traditional Democratic coalition, and that's still a very powerful one," said Zelizer.