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

The six-state contest Tuesday marked the first time voters weighed in on the primary since it effectively narrowed to a two-person race between Sanders and Biden. And the first four states on Tuesday went to Biden, a dramatic reversal for a campaign that appeared on the brink of collapse just two weeks ago.

The former vice-president won Michigan, the biggest prize of primary voting on Tuesday, extending his lead over main rival ,Senator Bernie Sanders.

Five other states – Washington, Missouri, Mississippi, Idaho and North Dakota – voted on Tuesday.

Mr Biden also swept aside Mr Sanders in Missouri, Mississippi and Idaho.

The Democrats’ next big election milestone is in a week’s time when 577 delegates are up for grabs.

To secure the nomination, a candidate needs the support of 1,991 delegates. Before Tuesday’s vote, Mr Biden had 648 to Mr Sanders’ 563.

The former vice-president had lagged behind his Democratic rivals in early voting states, but rebounded after big wins on Super Tuesday and endorsements from several former nomination rivals.

Thanking supporters from Philadelphia on Tuesday evening,  Biden said Democrats, including supporters of Bernie Sanders, could beat Donald Trump.

“It’s more than a comeback, this campaign,” he said. “It’s comeback for the soul of this nation.”

“Tonight we are a step closer to restoring dignity and honour to the White House,” Mr Biden added.

“We have to lead the world again,” the former vice-president said. “Donald Trump’s America First policy has made America alone.”

Senator Sanders, 78, enjoys widespread support among young voters but has so far failed to turn that into electoral success.