Hill Harper, running for an open U.S. Senate seat in Michigan, returned to Los Angeles earlier this month, where the actor headlined a fundraiser co-hosted by John Legend, as well as other figures including Vivica A. Fox and Mike Tollin.

The event was a needed boost to the actor and activist’s campaign: In the Democratic primary race, Harper is facing Rep. Elissa Slotkin (D-mi), who has a wide lead in polls and in her campaign war chest. She had more than $6 million cash on hand as of the end of last year, dwarfing that of her rivals. Harper had almost $150,000.

Harper, best known for his roles in The Good Doctor and CSI:ny, is largely running to the left of Slotkin, a former CIA analyst, and obviously hoping for a breakthrough. He also has relayed what Barack Obama, a classmate at Harvard Law School, told him that as hard...