Voters in Michigan and five other states headed to polls early Tuesday in the latest slate of primaries that will decide whether Joe Biden or Bernie Sanders will face President Donald Trump in November.

Idaho, Mississippi, Missouri, North Dakota and Washington state also vote Tuesday.

But Michigan, with its large delegate haul and its status as a swing state, is the day's top prize.

Biden has so far got 670 delegates and Sanders has 574.

To secure the nomination, a candidate needs the support of 1,991 pledged delegates. If no candidate has reached this number by the party's national convention in Milwaukee in July, it is known as a contested or brokered convention. The nominee is then chosen in a more complex voting system that also involves unpledged delegates – known as 'superdelegates' – who can vote their conscience. 

Sanders needs a strong showing in Michigan to remain viable as a candidate, and he and Biden have barnstormed the state in recent days in bids to broaden support and turn out the vote.

The two veteran politicians -- one a centrist, the other a leftist -- are locked in a duel for the Democratic nomination as all other major contenders have fallen by the wayside, many of them endorsing the surging former vice president Biden.

Sanders, who performed well in the three early voting states, is suddenly the underdog after Biden scored a decisive victory in South Carolina and then won in 10 of 14 states last week, on Super Tuesday.

Michigan had voted Democratic in the six previous presidential elections but Trump snatched it in 2016 by wooing disaffected workers, particularly in manufacturing industries like automaking.