Bernie Sanders Won Michigan in 2016. Tuesday’s Primary Looks Much Tougher.

Some of the more Clinton-friendly precincts from 2016 may be even more hospitable to Mr. Biden now that Michael R. Bloomberg, the former mayor of New York, is out of the presidential race. Polls taken while Mr. Bloomberg was still running showed Mr. Biden leading but with the former mayor of New York, who poured tens of millions of dollars into the state, also receiving some votes.

Yet now that Mr. Bloomberg has withdrawn and endorsed Mr. Biden, some of his supporters appear poised to migrate to the former vice president; a Detroit News survey taken before Super Tuesday found 49 percent of Mr. Bloomberg’s supporters indicating that Mr. Biden was their second choice, while just 18 percent said Mr. Sanders was their second choice.

These Bloomberg-to-Biden voters could prove especially crucial in the Detroit area, which was where Mrs. Clinton ran the strongest. In suburban Oakland County, for example, Mrs. Clinton won by about five points in 2016. But after Mr. Biden’s commanding margins in similar upscale communities on Super Tuesday, many Michigan Democrats expect him to win by far more there.

“Those women are voting for Joe Biden against Bernie Sanders and they’ll vote for him again against Donald Trump,” State Senator Adam Hollier said of the suburban women who helped power the Democrats in the midterms, and who surged to the polls last Tuesday.

Just as worrisome for Mr. Sanders, the same Detroit News poll had Mr. Biden winning 40 percent of black voters while Mr. Sanders was taking just 16 percent.

Mr. Hollier, who initially supported former Mayor Pete Buttigieg of South Bend, Ind., represents a legislative district in and around Detroit that includes a number of working-class black neighborhoods as well as some of the most affluent enclaves in the state. He said, “everybody in my circle is coalescing around Joe Biden.”

At first, he said, black and white voters alike were “looking for President Obama — but instead we went with the one we’ve always felt comfortable with.”

Lisa Lerer contributed reporting from Dearborn, Mich.