BOSTON — On Boston’s last day as a potential Olympic host, Chris Dempsey ate lunch at an Irish pub across from City Hall and waited for the bid for the 2024 Summer Games to crumble.

A waitress recognized Dempsey, the co-chair of the opposition group No Boston Olympics, from last week’s televised debate. A customer walked by and applauded, but his attempt at a high five was rebuffed.

“We’ll see,” Dempsey said.

It was about two hours after Mayor Marty Walsh refused to cover Olympic cost overruns, dooming the bid, and two hours before the USOC withdrew its support. Dempsey explained his hesitation in a truly Bostonian way: Referring to the stolen base that helped propel the Red Sox to the 2004 World Series, he said, “I want to make sure there’s no Dave Roberts-style comeback.”

Later Monday afternoon, the USOC announced that it was, indeed, backing away from Boston in what was described as a mutual agreement to give the United States hope of hosting its first Summer Games since 1996. Officials said they would explore other cities, with the most likely being Los Angeles.

Dempsey said he hoped the experience in Boston would change the way the USOC and the IOC do business. Otherwise, he said, “It’s hard to imagine smart cities and countries getting involved.”

Formed in a Beacon Hill living room in November 2013, when the prospect of bringing the Summer Games to the Athens of America was only slightly more realistic than it is now, No Boston Olympics grew into the most visible and sensible opposition to the plan to host the 2024 games.

Tapping into the city’s notorious negativity but also seizing on a growing resentment over the way international sports are run, the group led the campaign to scuttle what it saw as an attempt by Boston’s power brokers to determine what’s best for the city without any input from the people the plans would affect.

“This never was a bid that boiled up from the bottom. It was always a top-down effort,” Dempsey said in a previously scheduled interview with The Associated Press that happened to fall in the middle of the bid’s collapse. “Boston is a thriving democracy, and a marketplace of ideas. People reacted negatively to the idea that this was sprung on them.”

The USOC chose Boston as the potential American host city in a secretive process in January over San Francisco, Los Angeles and Washington, D.C.

But the bid was in trouble from the start.

Poll numbers showed a consistent majority of Boston residents opposed hosting the games. Few believed the local businessmen leading the bid could deliver without billions in taxpayer subsidies. Many feared another expensive and disruptive project like the Big Dig. The unrelated indictment of seven top international soccer officials was seen as evidence that international sports authorities can’t be trusted.

Four different men took turns as the face of Boston 2024, the privately run and financed bid committee, but No Boston Olympics hammered away at the same points: That planning for the games would distract the city from more important needs, and that a lack of transparency undermined any promises that might otherwise be welcomed.

Every Boston 2024 announcement was followed by a No Boston Olympics statement, usually pointing out gaps in the financial projections, or simply noting that the plan still required the three most expensive venues to be built from scratch. Led by Dempsey and co-chairs Liam Kerr and Kelley Gossett, No Boston Olympics established a presence on social media but also excelled at old-style mobilization, bringing their supporters to public meetings while holding town halls of their own to get their message out.

That message: Hosting the Olympics is a bad way to grow a city.

“The process is set up to lead to poor outcomes for host cities,” Dempsey said as Gossett checked her phone for final word on the bid. “There are too many places where the needs of the IOC are opposed to the needs of the taxpayer back home.”

Though the mayor denigrated the opposition as “10 people on Twitter” on Monday, No Boston Olympics accumulated about 3,700 Twitter followers and 4,500 likes on Facebook by the end. The group also forced old-fashioned debates and worked the traditional media despite a budget of about $10,000.

Average donations to the non-profit were about $100 from an undisclosed list of contributors that Dempsey said were in the hundreds. That was still more donors than the official bid group, which received an average contribution of $74,000, he said, and outspent the opposition by a 1,000-to-1 ratio.

“They can make a video of David Ortiz, and it looks great,” Dempsey said, referring to the belated attempt to enlist local sports stars in support of the bid. In response, No Boston Olympics spent $100 to print out a giant novelty check made out to the IOC, with the amount blank and the signature reading “Taxpayers.”

With the end of the 2024 bid, Dempsey said No Boston Olympics “will go away.” A Harvard Business School graduate and Bain consultant who worked with Boston 2024 CEO Rich Davey at the Massachusetts Department of Transportation, Dempsey said he had no plans to run for office.

But he said that the group’s three leaders hope to turn the public energy generated by the debate into something else.

“Our organization was focused on opposing Boston 2024,” he said at a news conference before heading to a local bar for a celebration. “At the same time, there has been a lot of energy from Olympic opponents that should be turned into something positive. So we will have to figure out how to do that.”