Hill Harper, an author and actor, on Monday entered Michigan’s 2024 Senate race, pledging to run to the left of Representative Elissa Slotkin, a moderate Democrat, in what is expected to be one of the most closely watched Democratic primary races in a 2024 presidential battleground state.

Mr. Harper, a first-time candidate known for his roles on “CSI: NY” and “The Good Doctor,” began his campaign with a message focused on expanding Social Security and access to affordable health care, as well as tackling income inequality and student debt. In an interview, he said he planned to position himself as “the most progressive candidate” in the race and would work to bring jaded and unheard voters back into the Democratic fold.

“It became clear to me that folks across Michigan don’t feel like they are being represented in Washington, D.C.,” he said, describing his conversations with people at farmers’ markets and union halls as he weighed his decision to run. The state’s Democratic leadership, which holds the governor’s office and both chambers of the Michigan Statehouse, he added, has “done a lot to move the state forward, but Washington is still broken.”

The Democratic primary is likely to be followed by a heated general election against a tense backdrop. Donald J. Trump won this industrial Midwestern state by nearly 11,000 votes in 2016, and lost it to Joseph R. Biden Jr. by more than 150,000 votes in his 2020 re-election bid. Mr. Trump focused on the voting in Michigan in his efforts to subvert the 2020 election.

As they head into the 2024 presidential cycle, Democrats will be concentrating on holding on to their midterm victories in Michigan in 2022, when protecting abortion rights galvanized the party nationwide.

Mr. Harper’s supporters believe that, as a Black progressive, he will be able to draw a coalition of liberal and Black voters. His campaign could particularly resonate with Black voters in a state where the debate over race and representation has raged in some corners: The 2022 midterm election left Detroit, the nation’s largest majority Black city, without Black representation in Congress for the first time in decades.

But Mr. Harper will face an uphill climb against Ms. Slotkin, a former C.I.A. analyst and three-term congresswoman who has built a track record as a seasoned campaigner and prolific fund-raiser. She has won three tough races for her House seat in a central Michigan district, which encompasses Lansing. Ms. Slotkin had more than $2.3 million cash in hand at the end of March, and is running on a platform focused on jobs and economic issues.

She was the first in her party to declare her candidacy for the seat being vacated by Senator Debbie Stabenow, a Democrat, and has the solid backing of establishment Democrats at the state and national level.

That hasn’t dissuaded Mr. Harper and a handful of Democrats from jumping in, including Leslie Love, a former state lawmaker, and Pamela Pugh, who serves as president of the Michigan State Board of Education.

For the Republicans, Nikki Snyder, a member of the State Board of Education, and Ezra Scott, a former Berrien County commissioner, are vying for the seat.