Vice President Kamala Harris used the last major speech of her campaign to unleash a fiery broadside against former President Donald J. Trump, calling her rival “consumed with grievance and out for unchecked power” and presenting herself as a fighter who would usher in a new generation of leadership.

In a speech in front of what her campaign said was around 75,000 people assembled at the Ellipse, a park just south of the White House in Washington, Ms. Harris sharpened her case, gesturing to the nation’s seat of power and bringing up the specter of a riot that had unfolded less than two miles to the east.

“In less than 90 days, either Donald Trump or I will be in the Oval Office,” she said, pointing back to the building behind her. “On Day 1, if elected, Donald Trump would walk into that office with an enemies list. When elected, I will walk in with a to-do list.”

But her speech had to bear more weight than just another attack on Mr. Trump. So on the 101st day of her improbable presidential campaign, Ms. Harris presented herself as a former prosecutor who had long worked for the public good — pausing to remind her audience that she had spent most of her career outside Washington and assuring her listeners that “not all good ideas come from here.”

Relaying her case in a way that was plain-spoken but forceful, Ms. Harris told the crowd that she would not be a perfect president but that she would govern with unity in mind, based on what she said was a “lifelong instinct to protect” people who had been abused or victimized.

But she also used the arc of history to make her case, saying that the country was born “when we wrested freedom from a petty tyrant” and that, over centuries, Americans had fought threats both foreign and domestic to preserve the promise of democracy.

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