Mr. Biden has had a remarkable two-week stretch: He entered the South Carolina primary confronting grave uncertainty about his future in the race, and concluded the night with an extraordinary jolt of momentum that propelled him to victory in 10 of the 14 states that voted three days later on Super Tuesday.
Now he faces favorable terrain in Mississippi, a state with a diverse Democratic electorate that Mr. Sanders has virtually conceded to him. He has also drawn large crowds in Missouri.
The biggest fight of the night for both men is shaping up to be Michigan, a large delegate prize where Mr. Sanders has focused much of his energy in recent days.
If Mr. Biden can roll to a decisive victory over Mr. Sanders in Michigan — as polls have suggested he might — there is the possibility that he may effectively wrap up the nomination at a time when Democratic voters are eager to turn their focus to President Trump, though Mr. Sanders’s next moves would be unclear.
If the race is narrower or if Mr. Sanders pulls off a victory, Democrats could head into a long primary slog.
What’s at stake for Sanders?
Mr. Sanders needs to dent the sense of inevitability of Mr. Biden’s nomination. That means a larger than expected win, with Washington and Michigan being the most likely of the delegate-rich opportunities.
Ideally, for the Sanders campaign, he would show improvement among black voters in the South and keep losses to a minimum in a state like Mississippi. However, considering Mr. Sanders pulled out of an event there this week, the campaign may see this as a pipe dream. Instead, flanked by figures like the Rev. Jesse Jackson and Representative Rashida Tlaib in Detroit, he is hoping to dent losses among black voters in Northern states.
But in the end, it really doesn’t matter why — or how. Mr. Sanders just needs to surprise.
Why is Michigan so important?
Michigan is the most closely watched state voting on Tuesday. That is in part because Mr. Sanders scored a surprise upset there that prolonged the Democratic primary race four years ago. And it is because Michigan is a Midwestern bellwether: If Mr. Sanders cannot revive his campaign there, he is unlikely to perform much better when Ohio, Illinois and Wisconsin vote in the weeks to come.
But Mr. Biden appears strong in Michigan. Every public poll has him leading by double digits, and the question now may be just how wide of a margin he will enjoy on Tuesday.
Watch three constituencies: African-Americans, college-educated white voters and the rural Michiganders who supported Mr. Sanders in large numbers in 2016.
The first two groups make up the former vice president’s Super Tuesday coalition and, with former Mayor Michael R. Bloomberg of New York out of the presidential race, are poised to support Mr. Biden again. And if Mr. Biden can make inroads with those rural voters, some of whom supported Mr. Sanders in 2016 because they were uneasy with Hillary Clinton, Michigan will hand him a decisive win.