“To be here is worth 1,000 Champagne bottles and firecrackers,”Ms. Harris told a reporter on New Year’s Day, surrounded by campaign detritus.

 Ms. Harris turned up in a dingy campaign field office, wearing a puffy coat and boots. Near a tangle of power cords, someone had propped an “African Americans for Obama” poster against a wall. In the midst of that bleak Midwestern winter, Ms. Harris was there to knock on doors for Barack Obama, the junior senator from Illinois mounting a long-shot bid for the presidency.

“To be here is worth 1,000 Champagne bottles and firecrackers,” Ms. Harris told a reporter on New Year’s Day, surrounded by campaign detritus. “It’s equal to that, in terms of the thrill, the excitement and the promise for tomorrow.”

Supporting Mr. Obama was a political risk. Ms. Harris was one of the rare Californians holding elected office — and one of few in the Democratic Party writ large — to endorse him for the presidency. Most of the party’s institutional heft had been thrown behind Hillary Clinton, a senator from New York who had a powerful surrogate in her husband, former President Bill Clinton.