The showing dispelled the argument that the former mayor of South Bend, Ind., made for months when asked about his dismal showing in polls of black voters — that winning performances among white Democrats in Iowa and New Hampshire would give a more diverse set of voters permission to believe in a 38-year-old most of them had never heard of a year ago.

“I just wish we had another month,” said J.A. Moore, a state representative from North Charleston who two weeks ago became the first black South Carolina state lawmaker to endorse Mr. Buttigieg. “If he’d been able to camp out here like he did in Iowa, I think the outcome would be a lot more favorable.”

Mr. Buttigieg’s showing here wasn’t for a lack of trying. He spent more money on South Carolina television advertising, $2.5 million, than anyone other than Mr. Steyer, whose campaign spent millions on black-owned businesses, staff members and political operatives. No candidate spent more days campaigning in South Carolina during the 2020 cycle than Mr. Buttigieg.

None of it worked.