Unionized UPS employees voted overwhelmingly to authorize a strike as contract negotiations continue, clearing the way for a potential work stoppage as soon as Aug. 1.

Some 97% of workers who cast ballots voted in favor of the move, Teamsters leaders said Friday, after more than a week of voting that preceded Tuesday night’s tentative deal on heat safety that would cover 340,000 delivery drivers and package handlers at the nation’s largest carrier.

Teamsters President Sean O’Brien said in a statement that the vote showed workers “are united and determined to get the best contract in our history at UPS. If this multibillion-dollar corporation fails to deliver on the contract that our hardworking members deserve, UPS will be striking itself.”

UPS acknowledged the vote outcome and noted that Friday’s strike authorization doesn’t automatically trigger a work stoppage.

“The results do not mean a strike is imminent and do not impact our current business operations in any way,” the company said in a statement. “We continue to make progress on key issues and remain confident that we will reach an agreement that provides wins for our employees, the Teamsters, our company and our customers.”

The decision comes days after union leaders and UPS reached a handshake agreement in which the company committed to phasing in air conditioning across its fleet of iconic brown delivery vehicles for the first time.

Drivers and labor advocates hailed the deal as an unexpected step forward on a key issue in the current round of labor talks.

“Folks are super excited,” said Zakk Luttrell, a UPS driver and union shop steward in Norman, Okla. “This is something that they said was not going to happen. We’ve heard for years it’s not going to be effective.”

UPS had long resisted calls to air condition its trucks and vans even as at least 145 of its employees have been hospitalized for heat illnesses since 2015, according to an NBC News analysis of Occupational Safety and Health Administration data. Luttrell hailed the shift as a long-awaited acknowledgment by the company that record summertime temperatures demand a change in approach.

“With the heat being what it is…it’s not just about what’s cost effective and efficient anymore,” he said, “it’s about keeping people alive.”

Amit Mehrotra, a managing director and research analyst at Deutsche Bank who covers the transportation sector, described the progress on heat mitigation as “one piece of the puzzle” that was “probably in the top five overall issues” in contract talks.

“It’s a drop in the bucket from a cost perspective for UPS, and it has an outsize quality-of-life benefit to the union, so I think it’s a win-win,” he said.

Mehrotra voiced optimism on the direction of progress talks overall, saying he expected the parties would “get this sealed up and done by the end of July” and avoid a strike.

A work stoppage at UPS would be the largest single-employer strike in U.S. history. Logistics experts say that even just several days of halted UPS deliveries would disrupt the flow of more packages than top rivals such as FedEx or the U.S. Postal Service could absorb, threatening to upend the back-to-school shopping season.

“UPS’s success is really tied to the success of the Teamsters, because what they do from a service perspective is really important,” Mehrotra said. “Now, the other side of that coin is the success of UPS is so critical to the Teamsters’ viability, because it’s really the only place that has seen massive Teamsters employment growth,” after other major unions’ membership declines have left UPS “literally the one oasis in this vast desert” for the labor movement.

He added, “I don’t know how a strike is not a lose-lose.”

While many union members at UPS cast their votes on the strike authorization before the heat safety deal was announced, some drivers said afterward that other big priorities remain. Luttrell, for his part, said “excessive” overtime demands are his main concern.

“We make good money because we have a union, but all of my time should not belong to this company,” he said.

Mehrotra said he expects UPS to close the gap on compensation issues, such as establishing wage parity between different classifications of workers, which he described as an “incremental cost” to the company.

Heat safety experts praised the preliminary agreement on air conditioning but cautioned that addressing the threat of extreme temperatures would take time.

“Even though there’s a lot of opening and closing of those doors, it will make it so those vans don’t continue to heat up and become ovens throughout the day,” said Juley Fulcher, an advocate at the consumer-rights nonprofit Public Citizen who focuses on heat safety.

But in part because the changes will first apply only to newly purchased vehicles, she said, “this is something that is not going to be an immediate solution for workers,” adding, “These fleets do take time to turn over.”

Some advocates and Teamsters leaders have also called for a more dynamic scheduling system that could better distribute driving routes on very hot days, reducing the number of packages each driver has to deliver.

“Work volume has to be a part of the discussion,” Fulcher said, “because when we’re talking about heat stress, the heat is coming from two sources — it’s coming from inside your body and outside your body.”

Seth Harris, a law and policy professor at Northeastern University who served as President Joe Biden’s top labor policy adviser, said progress on heat safety at UPS could have broader ripple effects.

“The tentative deal to guard against heat hazards is going to put tremendous pressure on UPS’s competitors to match those standards or exceed them,” he said. “Drivers looking for jobs are going to want to know whether their employer is going to take care of them and keep them safe.”

Already, though, the concessions have jolted UPS workers and their allies with a dose of optimism.

“We’re so, so excited you have no idea,” said Theresa Klenk, a nurse and the wife of a New Jersey UPS driver who suffered severe heat illness on the job in 2016, leading her to launch a petition for air-conditioned trucks that has since garnered more than 1.3 million signatures.

The newly announced changes, if ultimately approved as part of a new contract are “huge,” Klenk said. “I think it’s a great start.’