It was a night of former presidents and would-be presidents, and the prime-time debut of Gov. Tim Walz, who was cheered by members of the high school football team he once coached. Democrats on Day 3 of their national convention also celebrated Minnesota, the home state of Mr. Walz, with a special tribute to the artist once known as Prince, and an appearance by Senator Amy Klobuchar.

Still, the night proved that celebrity still matters in politics and beyond. Here are some highlights:

To Democrats, they’re no longer “deplorables.” They’re your neighbors.

Throughout the night, Democrats channeled their inner Ned Flanders, the “Simpsons” character known as the most neighborly neighbor in Springfield.

Speaker after speaker stressed that, even as they thrashed Donald J. Trump inside the convention hall, they did not view his followers as their enemies.

“That family down the road — they may not think like you do, they may not pray like you do, they may not love like you do, but they are your neighbors,” Mr. Walz said in his speech accepting the Democrats’ vice-presidential nomination. “You look out for them, and they look out for you.”

And Oprah Winfrey said before that: “We are not so different from our neighbors — when a house is on fire, we do not ask about the homeowner’s race or religion, we do not wonder who their partner is or how they voted.”

And there was former President Bill Clinton, urging Democrats to embrace neighborly debate instead of online smack talk with those of opposing beliefs. “I urge you not to demean them, but not to pretend you don’t disagree with them if you don’t — treat them with respect, just the way you would like them to treat you.”

It was in the final weeks of the election eight years ago when Mr. Clinton’s wife, Hillary Clinton, described some of Mr. Trump’s supporters as falling into a “basket of deplorables,” people who were “racist, sexist, homophobic, xenophobic and Islamophobic.” It became a rallying cry for Mr. Trump’s followers, and her party clearly learned a lesson from it.

The party sent a message of generational change.

Mr. Clinton, his voice softer, his hair thinner (but still impressive for his age) played the part of chief baton passer.

More than 30 years after he rocked his own convention stage as the face of the future, he portrayed himself as a charter member of Generation Old, ready to be led by Ms. Harris, who is just one year shy of being of Generation X.

“Lord, I’m getting old,” he said at one point from the stage. “But here’s what I want you to know — if you vote for this team, if you can get them elected and let them bring in this breath of fresh air, you will be proud of it for the rest of your life.”

And he sought to drag Mr. Trump with him to the political retirement home. Noting that he had just turned 78, Mr. Clinton said, “The only personal vanity I want to assert is that I’m still younger than Donald Trump.”

Mr. Clinton left the stage to his old optimistic anthem, Fleetwood Mac’s “Don’t Stop,” a song that felt as culturally distant from the moment as a Frank Sinatra single.

But the real sign of change came from the appearances of several next-gen party stars, like Pete Buttigieg, the transportation secretary, and two governors, Josh Shapiro of Pennsylvania and Wes Moore of Maryland.

As Representative Andy Kim of New Jersey, 42, who is running for a Senate seat this year, put it in his remarks: “There is a hunger right now in the country for a new generation of leadership to step up.”

The party’s strivers tried for a breakthrough moment. But Oprah Winfrey outshone them all.

The 2004 convention speech given by Barack Obama, when he was a state senator from Illinois, set him on a path to the presidency, and it is the electrifying moment many ambitious Democrats seek to match.

So, there were Mr. Shapiro and Mr. Moore and Mr. Buttigieg channeling their own inner Obamas as they worked to get the crowd on its feet with soaring calls to better angels and promises of hope and change.

But it was Oprah Winfrey, whose surprise appearance convention organizers managed to keep secret, who came the closest.

In a purple pantsuit, she struck singsong notes of inspiration and thundering beats of defiance, belting out the Harris campaign’s new “Freedom” message as if she was giving everyone in the audience a car.

“And let us choose the sweet promise of tomorrow over the bitter return to yesterday,” she said in a climax that sent the audience to its feet like no one else had. “We won’t be sent back, pushed back, kicked back, we are not going back.”

Mr. Walz fully embraced his subordinate role.

Tim Walz proved why Harris chose him. He kept it short, and, even as he introduced himself to millions of Americans for the first time, heeded the cardinal rule of vice-presidential protocol: never upstage the boss.

His speech ran for a crisp 16 minutes, about half the length of Mr. Clinton’s expansive address. He knew his role, and was sure to praise the woman who had plucked him from semi-obscurity and brought him to the biggest stage in global politics.

“No matter who you are, Kamala Harris is going to stand up and fight for your freedom to live the life that you want to lead because that’s what we want for ourselves,” he said, adding: “Kamala Harris is tough. Kamala Harris is experienced. And Kamala Harris is ready.”

Ms. Harris calls her running mate “Coach Walz” and he signaled his understanding of the role, with his sole focus getting his star quarterback, Ms. Harris, to the end zone.

“Our job for everyone watching is to get in the trenches and do the blocking and tackling,” he told the crowd. “We’re going to leave it on the field.”

Democrats know they have trouble ahead.

The giddiness was off the charts inside the convention hall. But the Harris team, perhaps worried about complacency, appeared to go out of its way to lace at least some speeches with reminders that Mr. Trump remains a stubborn and fierce opponent. The good times most certainly would not, could not, last forever.

“We feel like a lot is off our shoulders, we are happy,” Mr. Clinton said. But, he added, “We’ve seen more than one election slip away from us when we thought it couldn’t happen when people get distracted by phony issues or overconfident.”

He knew this much: “This is a brutal, tough business.”