Paul Mitchell, who runs a bipartisan political data company based in California, said the outcome would be decided even earlier than the March 3 calendar date would suggest because of the state’s early-voting rules. He projected that 5 percent of California primary votes would be cast before the New Hampshire primary, 25 percent before Nevada and a whopping 45 percent before South Carolina.

“The idea that you’re going to sail into California on a win that is late in those first four, it’s kind of fiction because such a huge portion of California will already have cast ballots,” Mr. Mitchell said. “They need to treat it as sometime between New Hampshire and Nevada.”

That could spell trouble for Ms. Harris, whose campaign has signaled a focus on South Carolina, the first state on the primary schedule with a heavily African-American population.

Ms. Harris is working to build a home-state advantage, holding events in her native Bay area as well as in Los Angeles while also counting on a battalion of supportive local Democratic officials to act as her surrogates while she spends much of her time in the first early states. She rolled out a number of new California endorsements ahead of the convention, including more than half of the Democrats in the State Assembly, and in her speech she torched President Trump over what she called his “pathological failure of leadership.” The California-based strategists running Ms. Harris’s national campaign have deep experience in the state, having overseen her past races as well as those of Mr. Newsom and a number of other Democratic lawmakers.

But she faces a challenge from candidates like Ms. Warren, who used the California convention to send a message of her own re-emergence, by holding the Friday rally in Oakland; buying a billboard in San Francisco calling to “break up big tech”; stuffing the convention hall with sign-wavers on Saturday; and lacing her stump speech with new lines that appeared aimed at Mr. Biden’s talk of working with Republicans.

“Some Democrats in Washington believe the only changes we can get are tweaks and nudges. If they dream at all, they dream small. Some say if we all just calm down, the Republicans will come to their senses,” Ms. Warren said. “The time for small ideas is over.”