“It matters who is in those rooms,” she said. “I demanded we reject the false choice that said you’re either tough on crime or soft on crime. Instead, I said we must be smart on crime.”

Ms. Harris burst onto the presidential stage with an announcement before a crowd of 20,000 in her hometown, Oakland, Calif., an early show of force in a race that began without a clear front-runner. She has fallen in recent polls, however, which has triggered concern within her campaign and among her supporters.

But those close to Ms. Harris say they are confident she is finding her stride. On Sunday, at a dinner for Iowa Democrats that several presidential candidates will attend, Ms. Harris will revive her focus on President Trump, according to an aide, and argue that she is uniquely positioned to “prosecute the case” against him. Her speech Saturday got several ovations among the older black crowd that gathered for the annual N.A.A.C.P. Freedom Fund dinner.

Ms. Harris was the first black woman to serve as California attorney general and the second black woman to serve in the United States Senate. The speech made a vigorous case for the power of racial representation, and she frequently used “we” to refer to the black community. Ms. Harris argued that because of her identity, background and experiences, her mere presence in the state’s law enforcement systems helped make them more just and equitable.