Delivering a jolt of enthusiasm to a party reeling from weeks of infighting, Vice President Kamala Harris rallied Democrats on Tuesday in Wisconsin and laid out a fierce argument against former President Donald J. Trump.

Ms. Harris vowed, in her first rally as the de facto Democratic presidential nominee, to defeat Mr. Trump by attacking him as a prosecutor would. She defined herself as a tribune of the middle class fighting against a tool of billionaires and as a champion of abortion rights against a man who would deny such rights to all Americans.

Ms. Harris offered a far more energetic denunciation of Mr. Trump than President Biden, in front of a crowd that her campaign said was the largest she or Mr. Biden had addressed since their re-election bid began over a year ago. She walked out to cheers to the tune of Beyoncé’s “Freedom,” which the singer had allowed her to use. As one attendee put it, the moment felt like a release of months of pent-up Democratic energy.

Interrupted several times by chants of “Ka-ma-la,” Ms. Harris demonstrated how Mr. Biden’s withdrawal from the 2024 race and her elevation have transformed a desultory, almost perfunctory campaign into a bastion of enthusiasm. She highlighted the $100 million in contributions since Sunday and took a victory lap for effectively wrapping up the Democratic presidential nomination within 48 hours.

“We have earned the support of enough delegates to secure the Democratic nomination,” Ms. Harris said. “I am so very honored and I pledge to you I will spend the coming weeks continuing to unite our party so that we are ready to win in November.”

The vice president recycled phrases from her 2020 presidential campaign, calling her bid “people-powered” and promising that, as president, she would prioritize the needs of the middle and working class over the desires of corporate interests and the wealthy.