CHICAGO — Democrats were giddy. After months of hand-wringing and pessimism before President Joe Biden stepped aside, they were finally excited again.

Among the delegates inside the United Center, there was one overriding thought.

We can’t lose.

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“She’s going to win in a landslide,” said Aleksy Federov, 36, a delegate from Washington state who recently became a citizen. Wearing a white blinking cowboy hat, Federov cited the “energy in the air,” adding, “It’s generational change in politics.”

“And there is Kamala Harris,” he said Thursday afternoon, hours before the vice president formally accepted the Democratic nomination for president at the convention in Chicago. “She brings the joy to the people.”

But in a series of high-profile admonitions during the four-day convention, several Democratic Party elders issued stern warnings for anyone believing the happy talk that Vice President Kamala Harris and Gov. Tim Walz of Minnesota are on a glide path to the White House.

“Never underestimate your adversary,” said former President Bill Clinton, whose wife knows all too well the bitter sting of losing to former President Donald Trump.

Former first lady Michelle Obama, who acknowledged in an address Tuesday night the renewed sense of hope among Democrats, still urged members of the party to temper their optimism with the reality that millions of Americans are determined to return Trump to office. And, she said, many people are willing to lie and distort the Harris record to make it happen.

“Yes, Kamala and Tim are doing great now,” she said. “We’re loving it. They are packing arenas across the country. Folks are energized. We are feeling good. But remember, there are still so many people who are desperate for a different outcome.”

“No matter how good we feel tonight, or tomorrow, or the next day,” she added, “this is going to be an uphill battle.”

The remarks from Bill Clinton and Michelle Obama underscored the dangers of this Democratic moment of joy and relief over Harris. Democratic voters who said for more than a year that the 81-year-old Biden was too old to run again now have their wish. In exchange, they have a 59-year-old candidate who has flipped the age argument on the 78-year-old Trump.

But polls remain tight. And other Democrats noted that there is a fine line between confidence and overconfidence, and that Republicans who gathered for their convention in Milwaukee a month ago to nominate Trump had been giddy, too.

The Republican convention put on display a party united behind its candidate at a moment when it seemed Democrats were faltering with an aging president. Words like “ebullient” and “triumphant” were thrown around. Trump’s campaign manager said the convention had gone “flawlessly.”

But fast-forward just a few weeks, and the terrain seems more difficult for Trump, with recent polls show him falling behind Harris in five out of seven key swing states. Sen, JD Vance of Ohio, the Republican vice-presidential nominee, has had a rocky rollout. Trump has struggled to land effective attacks against Harris after complaining that he had planned to run against Biden and the switch of nominees was not fair to him.

Sen. Chuck Schumer of New York, the majority leader, said in an interview that he is concerned that some people in his party are counting too much on a poor performance by Trump as the campaign heats up in the fall.

“There’s also a kind of feeling that Donald Trump will continue to screw up,” Schumer said this week. “We can influence it, but that’s not in our hands. If he doesn’t screw up, this race is neck and neck.”

In her remarks, Michelle Obama identified another concern: that Democrats will demand too much of Harris and Walz.

“The minute something goes wrong, the minute a lie takes hold, folks, we cannot start wringing our hands,” she said. “We cannot get a Goldilocks complex about whether everything is just right. And we cannot indulge our anxieties about whether this country will elect someone like Kamala, instead of doing everything we can to get someone like Kamala elected.”

Obama did not specifically refer to criticism of Harris at the convention for refusing to allow a pro-Palestinian speaker. A handful of delegates and protesters have accused Harris of embracing Israel too closely after attacks that have killed tens of thousands of civilians in the Gaza Strip.

By Thursday evening, the convention floor was buzzing as delegates milled about in the hour before programming kicked off on the final night. Nearly every walkway was mobbed, making moving through the floor laborious. But no one appeared bothered. Several delegates said they remained excited, but had also taken the sobering messages to heart.

“In my mind, with all my confidence, she is winning,” said Roxanne J. Persaud, a state senator from New York and a delegate. “But we still have a lot of work to be done. Yet, if every person does what they’re supposed to do, we are going to win this.”

For Christine Bartlett-Josie, a 55-year-old Connecticut delegate, the energy in the arena had a similar feel to Barack Obama’s 2008 convention, when he filled a Denver football stadium in a march to a decisive presidential victory that November.

“We can’t take anything for granted, and in ’08, we were mobilized and completely organized,” she said. “If, in the next 73 days, all this energy is transferred to people on the ground, we are going to win.”

Andrew Ashiofu, 41, a delegate from Washington state, was confident.

“Right from the Sunday when she announced, we’ve seen it,” he said. “We’ve seen people all coming together, the unity across all people. We’ve seen it in the way she’s raising funds, people donating massively to her. We’ve seen it in the polls. We’ve seen it everywhere. We’re seeing it right now.”

“America,” he predicted, “is united to make sure Kamala Harris wins in November.”

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