Former US vice president Joe Biden, flanked by his wife, Jill, addresses supporters after big primary wins in Michigan, Mississippi and Missouri (AFP)

Joe Biden moved a step closer on Tuesday to a November showdown with Donald Trump, notching up big primary wins over Bernie Sanders, his sole remaining challenger for the Democratic presidential nomination.

Biden, 77, the centrist former vice president, scored a resounding win in Michigan, where Sanders, 78, the leftist senator from Vermont, had hoped to make a stand.

He also cruised to victory in Missouri, Idaho and Mississippi.

The race was too close to call in Washington, the other major prize at stake, while North Dakota was also still counting votes.

Addressing supporters in Philadelphia as his victory took shape, Biden struck a presidential tone as he declared on national television that he would lead "a comeback for the soul of this nation."

"We're bringing this party together," he said. "Tonight we're a step closer to restoring decency, dignity and honor to the White House."

Biden reached out to his sometimes bitter rival Sanders and his supporters, telling them "we share a common goal."

"Together we'll defeat Donald Trump," he said.

Biden's huge victory in Mississippi, where he won around 80 percent of the vote, reflected his popularity among strategically vital African-American voters. He defeated Sanders by 25 points in Missouri.

His win in Michigan, the Midwestern industrial state which could be a key battleground in November, was a major setback for Sanders, who narrowly beat Hillary Clinton there four years ago.

Barring surprises, Biden now has a clear path to becoming the Democratic standardbearer in a potentially bruising and deeply divisive matchup with Trump.

The main question remaining 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.

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