Yet, even to some Harris allies, her decline is more predictable than surprising. In one instance after another, Ms. Harris and her closest advisers made flawed decisions about which states to focus on, issues to emphasize and opponents to target, all the while refusing to make difficult personnel choices to impose order on an unwieldy campaign, according to more than 50 current and former campaign staff members and allies, most of whom spoke on condition of anonymity to disclose private conversations and assessments involving the candidate.

Many of her own advisers are now pointing a finger directly at Ms. Harris. In interviews several of them criticized her for going on the offensive against rivals, only to retreat, and for not firmly choosing a side in the party’s ideological feud between liberals and moderates. She also created an organization with a campaign chairwoman, Maya Harris, who goes unchallenged in part because she is Ms. Harris’s sister, and a manager, Mr. Rodriguez, who could not be replaced without likely triggering the resignations of the candidate’s consulting team. Even at this late date, aides said it’s unclear who’s in charge of the campaign…

With just over two months until the Iowa caucuses, her staff is now riven between competing factions eager to belittle one another, and the candidate’s relationship with Mr. Rodriguez has turned frosty, according to multiple Democrats close to Ms. Harris. Several aides, including Jalisa Washington-Price, the state director in crucial South Carolina, have already had conversations about post-campaign jobs.