As Mr. Biden now attempts to leverage his Super Tuesday success and build momentum, Mr. Sanders may face even longer odds in Michigan than he did in 2016. The state that Mr. Sanders last week called “very, very important” suddenly looks forbidding for him.

Mr. Biden, despite having a thin operation in Michigan, appears likely to do well with black Democrats and college-educated white voters, two groups that handed him decisive margins in Virginia, North Carolina and several other states on Super Tuesday. And the exit polling and voting trends in some of those states indicate that Mr. Sanders has declined in strength with working-class white voters, who, uneasy with Mrs. Clinton in 2016, delivered him landslide wins across much of central and northern Michigan that year.

Michigan, with its 125 delegates, is the most populous state to vote on Tuesday, and it is the first of the big Midwestern battlegrounds to cast ballots — a general election trophy that President Trump painfully pulled from the Democratic column in 2016 with a narrow win. But Michigan also could amount to a bellwether for the rest of the Democratic primary race this spring.

With Mr. Biden appearing strong in the South and Mr. Sanders winning in the West, the industrial Midwest could effectively determine the nomination. And if Mr. Sanders can’t win in Michigan, he may struggle when Ohio and Illinois vote on March 17 and Wisconsin on April 7, while also undercutting his claims about expanding the electorate in some of the most pivotal general election swing states.