She began each of the rallies, titled “Dude Gotta Go,” with a lengthy discussion of the president’s purported crimes, and concluded by talking directly about the idea of “electability,” which she described as the “elephant” in the 2020 race, and the fear that the country was not ready to elect a black woman as president.

“Let’s have some real talk,” Ms. Harris said in Reno. “This is not a new conversation for me. This is a conversation that I’ve heard every time I’ve ran a campaign and — here’s the operative word — won.”

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In strategic terms, her campaign’s choice to pivot to Iowa is a concession that its initial theory of the presidential race was at least partially incorrect. From the start, her longtime advisers mapped out a national strategy that highlighted primaries with more diverse electorates — such as South Carolina, Nevada and California, her home state — while conserving resources often used in early states for television advertisements in later ones.

It was a risky bet made with California confidence, that they would not only boost a candidate with the potential to make history but also do so without prioritizing Iowa and New Hampshire, a strategy that is unproven on the national stage and a break from Barack Obama’s successful playbook in 2008.

But after the fleeting high of Ms. Harris’s much-talked-about exchange with Mr. Biden over segregation and school busing in the first Democratic debate, it became clear that several assumptions underpinning that national strategy were not in line with the current moment.

Mr. Biden’s support soon bounced back, fueled by perceptions that he was best suited to defeat Mr. Trump, which remains the obsessive top priority for most Democratic voters. Mr. Biden’s strength with black voters has been particularly difficult for Ms. Harris to match, and some of her own supporters from within the Congressional Black Caucus told her privately they felt the debate moment had crossed a line.

Ms. Warren and Mr. Sanders have also dictated the policy terms of the primary and the debates, pulling Ms. Harris between the dueling worlds of progressives advocating structural change and Mr. Biden’s yesteryear centrism. She prefers neither.