For weeks, the signs did not look good for Kamala Harris in Michigan. Literally. A digital billboard on the side of a barn, which I saw while driving to Grayling, read, “Willie Brown Endorses Kamala,” a farmer’s snarky reminder of a chapter of Ms. Harris’s life — when she briefly dated the speaker of the California Assembly — that I’m pretty sure she’d prefer voters forget.

Even in Detroit, usually friendly territory for Democrats, I bumped into haters. The leader of a well-to-do neighborhood association told me that the way Ms. Harris got nominated resembled “entrapment.” And, at a street fair, I chatted with Tamika Daniels, an activist who was working to register formerly incarcerated people to vote. Ms. Daniels expressed skepticism about Ms. Harris because the vice president had once been a prosecutor.

There was no sign of such skepticism at the Harris rally near the Detroit airport last Wednesday, where roughly 15,000 excited people waited hours to see her. The crowd included recovering Republicans who had never been to a political rally before, United Autoworkers members in matching red shirts and Black sorority sisters dressed head to manicured toe in pink and green. Some of them mentioned experiencing the same magic they’d felt during Barack Obama’s 2008 presidential run, as they pondered the possibility of breaking another barrier — this time, the first female president. “I missed out on Obama,” Sheila Sigro, who runs the Wayne County beauty pageant, told me. “I didn’t want to miss out on history again.”

Those comparisons are both inspiring and worrying. Inspiring because it does matter that such barriers are broken, and worrying because it can tempt Democrats to focus on style and symbolism over substance. This risk is most evident, and most significant, on the issue of Gaza. There is perhaps no issue that divides the Democratic Party more than the U.S. government’s support for Israel’s retaliation following the brutal attack by Hamas on Oct. 7. If the Harris campaign is unable to address this thorny issue in a way that feels like substance, then Democrats may not get the unity they’ll need to win in November.

Nowhere is this question more salient than Michigan, a must win-state and home to one of the largest Arab American communities in the country. Arab Americans turned away from the Democratic Party in large numbers, outraged that President Biden was spending their tax dollars to buy bombs that were killing their loved ones. To turn this outrage into political power, two Detroit-based Democratic organizers, Abbas Alawieh and Layla Elabed, co-founded a movement in Michigan to convince people to vote “uncommitted” in the Democratic primaries as a way to show their displeasure with President Biden and demonstrate their electoral strength. It quickly grew into a national effort.