Heather Ahern Huish was so elated when she read online that President Biden was endorsing Kamala Harris for the White House that the English teacher in rural Maine did something she hadn’t done in years. She made a political contribution, clicking on a Harris  Facebook  ad and donating $100—about the same amount she spends on groceries each week.

“I was just so swept up in the moment,” said Ahern Huish, who is 56 and lives with her husband. “There was, like, zero hesitation.” Later, she thought she should have looked closely at the donation link to make sure it wasn’t a scam.

Democratic donors have gone wild for Harris’s whirlwind bid for the presidency despite her tepid polling for years, invigorated by what they think is an Obama-esque opportunity to create generational change in the Democratic Party—and elect the first female president. While Biden’s fundraising efforts sagged for weeks amid questions about his age and fitness for office, his decision to leave the race jolted Democratic donors big and small into action; Harris later reported she had raised more than $200 million in her first week. Many big-money supporters will be piling into suites at the United Center and in Chicago’s luxury hotels this week as Harris accepts her party’s nomination at the Democratic National Convention .

While Biden typically charged $10,000 for a photo of himself with a donor, Harris recently charged $50,000 for such a ticket at her first big fundraiser in San Francisco—and quickly sold out. More than 6,000 people have handed over a $10 donation for an abstract “Kamala” quilting pattern made by a couple in Illinois. The haul from a “White Dudes for Harris” call left its organizers dumbstruck.

“If you had asked me if I would be someone who raised $4.5 million for a presidential campaign before this, I would have told you, ‘There is no chance in hell,’” said one of them, Ross Morales Rocketto.

Supporters are racing to capitalize on the moment. Harris’s husband, Doug Emhoff, jumped in to appear at fundraisers that were originally scheduled to feature first lady Jill Biden, stopping at summer-vacation spots such as Martha’s Vineyard and New York’s Fire Island to glad hand donors.

In the coming days, Emhoff will continue to traverse the country along with other top surrogates such as Michigan Gov. Gretchen Whitmer , North Carolina Gov. Roy Cooper and former House Speaker Nancy Pelosi . Earlier this summer, Donald Trump had more cash in his campaign account than Biden, an advantage that Harris has, at least for now, erased.

Some caution that sheer enthusiasm might not be enough as the campaign faces down three grueling months of competing against Trump.

“As hopeful and happy as we feel,” Democratic megadonor Susie Tompkins Buell wrote in a recent newsletter, “Vibes are not votes.”

The Fire Island crowd

In the days following the June 27 presidential debate, when Biden delivered a faltering performance, Democratic donors in New York’s business community threatened to withhold money, sending an informal but powerful signal to Biden to step aside. One of those donors was Marius Meland, an entrepreneur who founded a popular legal news service. Meland had agreed to host a Biden fundraiser on Fire Island, where he lives part-time with his husband.

Protocol dictated that Meland make a donation to the Biden campaign as the early August event approached, but Meland held off writing the check while he watched the national conversation over Biden’s fitness for office play out.

“I feel a little awkward talking about it because I don’t want to make anyone feel embarrassed. But it happened,” Meland said.

With Harris the nominee, the soiree was back on track. And so, on a recent Friday afternoon, a crowd of predominantly gay men gathered around Meland’s pool for a fundraiser featuring Emhoff and Chasten Buttigieg, the husband of Transportation Secretary Pete Buttigieg .

Guests ate hot dogs and hamburgers and drank rose and white wine. The event sold out, spurring the organizers to issue extra tickets and raise what they described as a record for the island, $325,000. The last record, one organizer said, was held by Cher.

“I have not felt so excited, personally, about a candidate since Obama,” Meland said. “I’m sure we’ll find reasons why she’s not perfect. But she does seem like an incredibly powerful leader.”

In a recent series of fundraisers Emhoff has headlined, he has riffed to the crowd about the importance of the election and told his oft-repeated story of missing the news about Biden dropping out because he was at SoulCycle. He has stopped once in Nantucket and twice in Martha’s Vineyard, where Harris’s sister, Maya, owns a home and stood in the crowd, the Vineyard Gazette reported .

Harris has spent the majority of her time on the campaign trail in recent days, though she did appear at a fundraiser  in San Francisco this month  at the four-star Fairmont hotel.

The event raised $13 million and drew roughly 700 people. Among those who attended were LinkedIn co-founder Reid Hoffman , who co-hosted the event, and Hollywood mogul Jeffrey Katzenberg .

“You could feel the electricity in the room,” Hoffman said.

Zooms that keep Zooming

In late July, political strategist-turned-businesswoman Jotaka Eaddy set up one of the regular Zoom calls she hosts for Black women in business, called Win With Black Women. Usually the calls attracted a few hundred people. But this time, Eaddy’s Sunday gathering became a vehicle for Harris fans looking for a way to show their support: More than 44,000 people joined the event with tens of thousands others listening in, according to Eaddy. She raised $1.6 million in roughly two hours.

It was the beginning of a viral moment: Eaddy’s call sparked dozens of other online fundraisers for Harris that have raised millions of dollars for her campaign. One recent Monday, Eaddy noted, a Harris supporter could choose from several Zoom meetings including “Comics for Harris” or Black church leaders for Harris.

“I remember talking to my friend and asking, ‘Am I going to go to church or the comedy show? Which will I do tonight?’” Eaddy said. “It’s the Zoom that keeps on Zooming.”

The successes have spilled onto other parts of the internet. A quilting company, Modern Quilt Studio, made a “Kamala” pattern to raise money for the campaign and saw thousands of people download its design, which quilters have turned into not just quilts but pillows, tote bags and banners.

Eaddy has now become an informal Zoom consultant, fielding daily inquiries from people looking to host their own online events for Harris. She and other volunteers provide advice on how to set up a video call and plan a program capable of entertaining tens of thousands of people.

Celebrity guests are increasingly common: The “White Dudes” call lasted three hours and featured a cameo from Jeff Bridges, who famously played “The Dude” in the 1998 cult movie “The Big Lebowski.”

Underpinning the fancy fundraisers and quirky Zooms are the small-dollar donors, many of whom say they had, until recently, felt fatigued by the state of politics and constant barrage of political fundraising texts.

But major news events appear to have reinvigorated the small-dollar base. After a Manhattan jury verdict found Trump guilty of 34 felonies, his campaign raised more than $52 million in 48 hours, according to the campaign.

Harris raised more than $81 million in the 24 hours after Biden dropped out of the race for president. According to a memo sent later by campaign chair Jen O’Malley Dillon , 64% of the people who gave were making their first donation of the political cycle.

The Maine teacher, Ahern Huish, doesn’t usually talk about politics with her students in Franklin County, a Trump stronghold. She once worked as an aide in the U.S. Senate, which left her feeling disillusioned by politics.

Harris’s sudden elevation, she said, gives her a “feeling of hope again.”

Write to Maggie Severns at maggie.severns@wsj.com