Wayne County

Wayne is the most populous county in the state and will be responsible for around 20 percent of the statewide vote. The candidates’ success there will largely hinge on their appeal to Black voters, particularly in Detroit, the largest city with a Black majority in the United States.

Progressives have sometimes struggled in Democratic primaries in Detroit — Senator Bernie Sanders, independent of Vermont, lost the city by wide margins in his two presidential bids, and Dr. El-Sayed placed third there in his 2018 run for governor.

The Congressional Black Caucus has endorsed Ms. Stevens, who has run ads featuring clips of former President Barack Obama praising her in 2018. (Mr. Obama has not endorsed either candidate.)

Wayne County as a whole is ideologically diverse. In the 2024 presidential primary, several jurisdictions had more voters cast a ballot for “uncommitted” than for President Joseph R. Biden Jr. Many were opposed to Mr. Biden’s support for Israel and its war in Gaza. At the same time, Mr. Biden won 86 percent of the vote in Detroit.

Several of the areas Mr. Biden lost — including the cities of Dearborn, Dearborn Heights and Hamtramck, which are home to high shares of Arab and Muslim voters — backed Mr. Sanders in 2020 and could favor Dr. El-Sayed, who strongly opposes the United States’ Middle East policies. The nation’s leading pro-Israel PAC has spent about $30 million in support of Ms. Stevens.

Oakland and Macomb Counties

The large suburban Detroit counties of Oakland and Macomb are emblematic of two very different kinds of shifts in the Trump era.

Oakland has some of the state’s highest concentrations of high-income, college-educated voters. Historically a Republican stronghold, the county has swung to the left in both state and local races. Ms. Stevens, whose seat covers a large swath of the county, is expected to do well there.

Macomb is decidedly more working class, with white residents without college degrees making up 56 percent of the adult population (in Oakland, it is 35 percent). Mr. Obama won Macomb in 2008 and 2012, but the county then voted for President Trump three elections in a row. Ms. Stevens and Dr. El-Sayed are both trying to appeal to voters there, many of whom work in the auto industry. Ms. Stevens has frequently referenced her work on an Obama administration auto rescue task force, while Dr. El-Sayed has secured the backing of the United Auto Workers.

Genesee County

Much like Macomb, Genesee County has trended sharply to the right in the Trump era, with Vice President Kamala Harris winning the county by only four percentage points in 2024 after Mr. Obama won it by nearly 30 percentage points in 2012.

Genesee has consistently supported more moderate candidates in Democratic primaries. Results from Flint, a majority Black city, may be another indicator of how Black voters are leaning.

Washtenaw, Ingham and Kalamazoo Counties

These counties, home to substantial shares of young voters, are important areas for Dr. El-Sayed. Polls have generally shown him running strongest with voters under 30 — a persistent trend for progressive candidates this cycle.

Washtenaw includes the college towns of Ann Arbor (University of Michigan), where Dr. El-Sayed lives, and Ypsilanti (Eastern Michigan University). Ingham includes East Lansing (Michigan State University), and Kalamazoo includes the city of Kalamazoo (Western Michigan University, Kalamazoo College).

Michigan’s college towns generated large percentages of protest votes against Mr. Biden in the 2024 Democratic primary.

Kent County

Kent County is among the fastest-growing counties in the state, due in large part to the growth of the Grand Rapids metro area. Kent, which has shifted leftward in recent presidential races, has been a favorable area in the state for progressives: Mr. Sanders won Grand Rapids in both 2016 and 2020.

Representative Hillary Scholten, a Democrat whose district includes much of the Grand Rapids area, has been a sharp critic of Dr. El-Sayed in recent weeks.