For decades, Kamala Harris has been bolstered by a tight-knit group of female donors who rose up with her in Democratic politics. And for weeks, even when she was still insisting that President Biden would be the party’s nominee, these allies began to make moves to make sure her historic campaign would not be built on the fly.

Quickly and quietly, her biggest supporters worked to rally support around her, creating enough momentum to effectively stamp out any opposition. They collected money, cut ads in advance and worked their networks to monitor the moves of other hopefuls.

On Sunday, when Mr. Biden announced his exit from the race and endorsed Vice President Harris, all the behind-the-scenes maneuvering appeared to pay off. The nation’s highest-ranking female officeholder, Ms. Harris rapidly picked up pivotal endorsements without attracting a single serious challenger. Money started flooding in. Ads began moving. And while some major Democratic donors remained on the hunt for a non-Harris candidate on Monday, their efforts were appearing increasingly futile by the hour.

Some Democratic donors who are not immediately supporting Ms. Harris — including former Mayor Michael R. Bloomberg of New York and Silicon Valley leaders such as Reed Hastings and Vinod Khosla — called for a competitive process, which Democratic donors and fund-raisers have been interpreting as something of a code to say they are not eager to support Ms. Harris against former President Donald J. Trump.

“The decision is too important to rush, because the election is too important to lose,” Mr. Bloomberg said on Monday.

The scale of the dissenters’ fortunes means that Ms. Harris’s team may not be able to fully ignore them, and now her allies’ task will be to silence — or at least muffle — remaining doubts. Donors and fund-raisers on Sunday and Monday were racing to put together briefings and calls with other donors, and to get pro-Harris ads on television, to capture whatever momentum Ms. Harris might harness.