As supporters walked into the first rally for Vice President Kamala Harris and Gov. Tim Walz of Minnesota, they were given light-up wristbands like the ones Taylor Swift’s fans wear at her concerts.

It was clear this was no longer President Biden’s campaign.

Seemingly overnight, the new Democratic ticket transformed the party’s presidential push from a dreary drag to — as Mr. Walz put it — “a joy.” The cross-country unveiling showcased an enthusiasm that more than a few Democrats say has not been on display since Barack Obama’s first presidential campaign.

The Biden-to-Harris swap has, in just three weeks, flipped the election on its head. Democrats can now avoid making the race a referendum on Mr. Biden — a contest that many worried they would lose — and instead cast it as a choice between a Harris-led future or a return to power by former President Donald J. Trump. Across the country, the vice president’s crowds have been chanting her refrain: “We’re not going back.”

Here are five striking themes that coursed through the first week of the Harris-Walz ticket.

Ms. Harris is campaigning as if she is winning, even as she cautions Democrats that she and Mr. Walz remain the “underdogs” against Mr. Trump.

In a remarkable development, Ms. Harris has turned Mr. Biden’s alarming poll numbers around. She now leads Mr. Trump in a national polling average and has pulled ahead in the must-win Northern battleground states of Michigan, Pennsylvania and Wisconsin, according to new surveys from The New York Times and Siena College.

“We’re going to do this!” she told the crowd at her Friday night rally in Arizona.

On the other side, Mr. Trump finds himself in an unfamiliar position: the back foot. He has challenged Ms. Harris to three debates, a move typically made by a candidate who fears defeat. (She has so far agreed to only one.)