Constance Lancelle, 22, of Milwaukee, was “definitely not interested in voting for Biden,” she said. But with Kamala Harris as the Democratic presidential candidate, she said, “I feel like politics have been a dream.”

Sierra Sanson, 23, of Medford, N.J., planned to vote for Jill Stein of the Green Party, or not at all. Now she’s thrilled to support Ms. Harris: “She’s a badass woman who I want to see succeed.”

Emily Baumel, 27, of Madison, Wis., hadn’t planned to vote for president, but will now vote for Ms. Harris: “I have a lot less dread; I like how much hope she’s giving people.”

Vice President Harris’s candidacy has reinvigorated many Democrats and independents — and particularly young women. Their newfound enthusiasm is evident in interviews, and in early signals from polls in swing states and nationwide.

As a group, young women were never going to support Donald J. Trump in the election, according to national New York Times/Siena College polls. But that had not always meant backing President Biden. His support among both women and the young (18 to 29), crucial to his 2020 victory, had been slipping before his exit from the race.

It’s still early in Ms. Harris’s candidacy, and the numbers of these voters included in polls are too small to make definitive conclusions, but so far she seems to be winning many of them back.

In Times/Siena polls this month of voters in six swing states, young women were, on average, 10 percentage points more likely to support Ms. Harris than they had been to support Mr. Biden in May.

Across those states — Arizona, Georgia, Michigan, Nevada, Pennsylvania and Wisconsin — 67 percent of young women now said they planned to vote for her, and 29 percent for Mr. Trump. A larger share of them support Ms. Harris in the Times/Siena polls than in any other age and gender group.

Young men were five points more likely to support Ms. Harris than Mr. Biden. But 53 percent planned to vote for Mr. Trump, and 40 percent for her. (Our companion article on them is here.)

Recent national polls from Ipsos and Marquette Law School found a similar shift among young women. “The historic summer we’ve lived through pulled some young women off the sidelines and into the election,” said Sarah Feldman, a senior data journalist at Ipsos. “These are more marginal shifts overall, but with an incredibly close election, these small decisive swings in support can matter.”

We called back young women who told Times/Siena pollsters this month that they supported Ms. Harris. Of the eight we reached for follow-up interviews, none said they had planned to vote for Mr. Trump, but they all had intended either to sit out the election, vote for a third-party candidate or begrudgingly vote for Mr. Biden.

“I would be proud to see her as president of this country, whereas with Biden, I was embarrassed by the idea,” Ms. Sanson said.

For many young women, Mr. Biden and Mr. Trump were the only presidential candidates they had been old enough to vote for, which they said left them feeling uninvested and discouraged.

Eloisa Gloria, 22, who is studying illustration at the Milwaukee Institute of Art and Design, said she had felt “disillusioned” by politics. Now, she is excited.

“This is the first time I have voted and genuinely felt positive about the candidate,” she said. “I think Kamala is incredibly intelligent and that she will care far more about our well-being than simply winning the election.”

She has been watching livestreams of Ms. Harris’s rallies, she said. She likes her ideas for paying teachers more, prioritizing abortion rights and addressing climate change.

“This was actually one of the first elections where I was like, honestly, I can’t shame people who don’t vote,” said Areli Herrera, 25, a nonprofit worker in Milwaukee. “I get it.”

That changed with Ms. Harris’s candidacy, she said: “I kind of felt a breath of relief.”

Ms. Herrera said Ms. Harris is more moderate than she is. She is wary of Ms. Harris’s background in law enforcement, and disliked how she cut short pro-Palestinian protesters who had interrupted her at a rally. But she said she appreciates that center-left ideas might help the Harris campaign win over moderate voters — and defeat Mr. Trump.

Ms. Harris’s choice of Tim Walz for a running mate bolstered her support: “She’s done what I could never do in my dating life — she picked the right white man.”

None of the young women interviewed mentioned Ms. Harris’s being a woman or person of color as a primary reason for their support, but some said it was a bonus.

Mina Kim, 27, in Duluth, Ga., has been uninterested in politics, and hadn’t planned to vote. But she has seen several things she’s liked from the Harris campaign, including efforts to cap the price of insulin and her support for reproductive rights.

Ms. Kim, who is Asian American, did not realize that Ms. Harris has South Asian heritage. “It wouldn’t be the reason to vote for her, but it’d be cool,” she said. “She’d be the first of many things.”

Overall, Gen Z women are “fiercely feminist and progressive,” more than any other group of Americans, said Melissa Deckman, a political scientist who is chief executive of P.R.R.I., a research firm, and author of the coming book “The Politics of Gen Z.”

Ms. Baumel, from Madison, works in health care and worries that Ms. Harris isn’t progressive enough, especially on the issues most important to her: student loans, health care and the war in Gaza.

But Ms. Harris has two major things going for her, Ms. Baumel said: “One, she’s not a 70-year-old man or older. Two, I really like her V.P. pick. Coming from the Midwest, I love to see Midwestern dads.”

“Will I vote for her in four more years?” she said. “I don’t know, but she’s worth a shot.”