Democrats head into their convention feeling optimistic -- and terrified: ANALYSIS

Democrats are jubilant, preparing for a political party this week as they descend on Chicago for their convention. They're also terrified.

In just a short time, Vice President Kamala Harris has energized her party as its de facto presidential nominee, riding a fawning social media wave and polling boost over President Joe Biden's numbers. Her formal acceptance of Democrats' nomination this week will cap just the latest moment in her weekslong "honeymoon" as she energizes a party that just a month ago was downright morose after Biden's ruinous June debate.

But the party is still gripped by profound dread over former President Donald Trump , who retains a viable path to victory and who overcame historical odds and his own stumbles to win the White House the last time Democrats were galvanized by a history-making female candidate.

Democrats are "more hopeful than I've seen them in a long time, more enthusiastic and energized than I've seen them in a long time, nervous because they remember 2016," said former Sen. Doug Jones, D-Ala., a Harris ally, recalling Hillary Clinton's campaign versus Trump. "You cannot take your foot off the gas. You cannot get overly confident just based on the fact that you feel good right now. This is going to be a tough race."

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Harris has leapfrogged Trump in 538's national polling average and in polling averages in some key swing states , albeit by a hair. She's filling arenas across the country, blitzing the campaign trail with Minnesota Gov. Tim Walz, her running mate. And she's revamped her campaign messaging to focus on an upbeat message of freedom, a departure from Biden's dark focus on Trump's supposed threat to democracy.

Poll after poll has shown a corresponding rise in Democratic enthusiasm and a drop in the number of so-called "double haters," a risk to Trump, who had previously been running against a party with a depressed base.

That's all set the stage for a convention this week that is being framed as something of a jamboree, instead of the relative political wake that had been planned for Biden, who had seen his path to victory narrow to the point that Democrats needed to squint to see it.

"I don't think we've felt national joy in a long time. Many of us felt it when Biden beat Trump. That's what it feels like to me," said one Democratic strategist with ties to Harris' team. "In talking to people, and when you just see the crowd, it just feels joyful. People feel excited."

But having a festival this week in Chicago is not the same as winning in November, and Democrats know it.

There are still policy headwinds ahead -- campus protests this fall over the war in Gaza could erupt again; inflation is cooling, but so is the labor market; immigration remains fertile ground for attack by Republicans.

And on top of all that looms Trump, who has struggle to find his footing since Harris emerged as his opponent but still commands fierce loyalty over his base and advantages on issues like the economy and the border.

The party's anxiety has become so well-known to the point of cliché, and this week, hanging over the Chicago convention, will be a pall.

"Never take [Trump] for granted. Never dismiss him," said longtime Democratic National Committee member James Zogby. "He'll be a mean son of a bitch, and he speaks to an anger and a sense of dislocation and being overlooked, being slighted, that is a significant percentage of the electorate that cannot be ignored."

Several Democrats who spoke to ABC News brought up the 2016 race between Trump and Hillary Clinton unprompted, recalling the shock they felt after feeling bullish for months that their candidate would come out on top.

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"You never get overconfident. We saw what happened in 2016 when people got overconfident, and we got four years of pure hell from Trump," said David Brand, a Democratic strategist and Harris ally.

"There's not a day that goes by we don't think about it," Brand added of the 2016 race.

Some Democrats, though, said that's not necessarily a bad thing.

Such fear -- which was not felt as prominently in 2016, they said -- could compel the party to work harder and prevent complacency, the enemy of a well-oiled campaign.

"It's healthy. We should be afraid," said Jamal Simmons, a longtime Democratic consultant and former communications director for Harris. "It's time to be sober."

Still, Democrats are making sure to enjoy their improved footing after Biden ended his campaign.

Instead of an 81-year-old incumbent whose mental fitness was the source of constant speculation, Democrats' standard bearer is now a 59-year-old prosecutor -- a fact that the party never fails to mention as it runs against a 78-year-old convicted on 34 felonies. That fundamental shift has Democrats at least breathing a sigh of relief, though still not remeasuring the drapes in the Oval Office.