Voters in Michigan and five other states are heading to the polls 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 today.
But Michigan, with its large delegate haul and its status as a swing state, is the day's top prize.
Mr Sanders needs a strong showing in Michigan to remain viable as a candidate, and he and Mr Biden have barnstormed the state in recent days in bids to broaden support and turn out the vote.
Mr Biden hopes to take a big step toward the Democratic presidential nomination.
The former vice president, who claimed the position of Democratic front-runner with a sweeping series of wins last week in Super Tuesday nominating contests, could build a formidable lead in the race to pick a challenger to Mr Trump with another round of decisive victories.
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With 125 delegates, Michigan is the largest prize of the six states voting today, when a total of 352 delegates to July's Democratic nominating convention will be up for grabs.
Since last week's successes on Super Tuesday, Mr Biden has moved into the national lead in polling and delegates, knocked out his remaining viable rivals except Mr Sanders and swept up another wave of endorsements from prominent Democrats and former rivals such as Senators Kamala Harris and Cory Booker.
Democrats who were worried that Mr Sanders' democratic socialist proposals to restructure the economy would doom the party to defeat in the November election have rushed to rally behind Mr Biden.
He has a double-digit lead in the four most recent polls taken in Michigan. But Mrs Clinton held a similar lead in Michigan polls over Mr Sanders, a senator from Vermont, before the 2016 primary, making predictions of the outcome this time even less reliable than usual.
Like his resounding win in South Carolina that was powered by overwhelming support from African Americans, Mr Biden was again lifted in Super Tuesday contests by strong support from black voters.
In 2016, about one-fifth of Democratic primary voters in Michigan and Missouri, and an overwhelming 70% in Mississippi, were black.
In Michigan, Sanders has pressed his criticism of Biden's past support for global trade deals such as the North American Free Trade Agreement. The trade pact is unpopular in Michigan, where it is blamed for costing jobs.
Mr has countered by recalling the Obama administration's decision to bail out the auto industry, an economic pillar in Michigan, and help its biggest city, Detroit, navigate a municipal bankruptcy.
A Biden breakthrough in Michigan, and big victories in Missouri and Mississippi, where he is heavily u, would put him in prime position to strike a virtual knockout on Mr Sanders in next week's 17 March contests, when the big states of Florida, Ohio, Illinois and Arizona all cast votes.
If Mr Biden retains a significant lead in delegates after next week's voting, Mr Sanders will be out of realistic possibilities to catch up. By the end of March, about two-thirds of the delegates up for grabs in the race will be allocated.
Heading into today's voting, Mr Biden leads Mr Sanders by 628-545 in pledged delegates, according to Edison Research. A total of 1,991 is needed for the nomination.
