Vice President Kamala Harris moved swiftly to assert herself as the de facto Democratic nominee for president on Monday, her first full day as a candidate, as virtually every potential remaining rival bowed out and she clinched the support of enough delegates to win the nomination.

The Associated Press said late Monday that Ms. Harris had secured the backing of more than the 1,976 delegates needed to capture the nomination in the first round of voting. The pledged support is not binding until the delegates cast their votes, which party officials said would take place between Aug. 1 and Aug. 7.

“When I announced my campaign for president, I said I intended to go out and earn this nomination,” Ms. Harris said in a statement. “Tonight, I am proud to have secured the broad support needed to become our party’s nominee.” She added, “I look forward to formally accepting the nomination soon.”

With barely more than 100 days until the election, Ms. Harris immediately pressed her case against former President Donald J. Trump during a visit to her new campaign headquarters, invoking her early career as a prosecutor who took on “predators” and “fraudsters.”

“Hear me when I say, I know Donald Trump’s type,” she said to cheers.

The vice president compared her day-old campaign to the civil rights and voting rights battles of the past, placing it on a continuum with “abolitionists and suffragettes.” And she said that Mr. Trump’s potential return would undo some of those victories and take the country backward.

“We are not going back,” she said.

Behind the scenes, Ms. Harris was moving just as quickly to take control of a sprawling political apparatus that just a day earlier had belonged to President Biden.