President Biden formally kicked off his re-election campaign over the weekend surrounded by 2,000 adoring union members in Philadelphia. It was, as Politico put it, his “happy place,” and he made no attempt to hide whose team he is on when it comes to labor relations.

“I look out in this crowd and I see a lot of old friends, a lot of folks, as they say in Claymont, Delaware, who brung me to the dance,” Mr. Biden said.

A day earlier, America’s largest federation of unions — the AFL-CIO — endorsed Mr. Biden. “There’s absolutely no question that Joe Biden is the most pro-union president in our lifetimes,” said the group’s president, Liz Shuler. “We’ve never seen a president more forcefully advocate for workers’ fundamental right to join a union. Now, it’s time to finish the job.”

Unfortunately for the president, one of the unions affiliated with the AFL-CIO, is poised to make his life much more difficult in the coming weeks. On the same day of the endorsement, the International Brotherhood of Teamsters’ 340,000 drivers voted overwhelmingly to authorize a strike at United Parcel Service if UPS and the union are unable to come to an agreement by July 31.

The president and chief executive of the American Trucking Association, Chris Spear, tells the Sun that a strike by the Teamsters would be costly for everyone. “If a strike or lockout occurs, many of the goods we depend on in daily life will not reach stores or our homes,” Mr. Spear says. “Product shortages and price spikes will occur. Businesses that rely on just-in-time delivery will suffer, as will their employees. Layoffs will follow.”

A Teamsters strike against UPS would be the biggest walkout by an American labor union since the 1950s, and harkens back to the fall of 2022 when the country narrowly averted a rail strike that would have crippled the nation’s supply chain and been a punishing blow to an economy still recovering from the Covid pandemic. Mr. Biden angered his union allies by forcing them to accept a deal to avert the strike, including two unions — the Brotherhood of Locomotive Engineers and Trainmen and the Brotherhood of Maintenance of Way Employes — that are also Teamsters. This time, however, he may meet more resistance to any such intervention.

Much of the blame for the current impasse is being laid at the feet of Sean O’Brien, who took over as president of the Teamsters in March 2022. In recent months, Mr. O’Brien and his lieutenants have taken an increasingly strident tone in negotiations with several trucking companies and promised in expletive-laden comments and tweets that they intend to do the same with UPS.

Referring to the company as a “white collar crime syndicate” during a rally in Boston in April, Mr. O’Brien said, “We are not going to take and accept what UPS gives us. We’re going to demand, take and punish if they don’t give us what we want moving forward.”

Among the union’s demands are higher pay, the removal of surveillance cameras from delivery trucks, more full-time jobs and the elimination of a two-tier wage system where newer workers are paid less than old-timers who do the same job. Last week, the union secured an agreement from UPS that calls for the installation of air conditioning in the company’s fleet of brown delivery trucks.

At a California rally two months ago, Mr. O’Brien told the gathering that he has been meeting regularly with staff at the White House to update them on progress in the talks, which began in April. During those meetings, he said, he warned the Biden administration against interfering in any potential strike against UPS as it did with rail workers. “I grew up in a tough neighborhood in Boston,” he said. “We had a code of honor. If two people are fighting in the street and you have nothing to do with it, you keep walking.”

Trucking industry officials, however, say the president may have to get involved whether he wants to or not and regardless of whether it annoys those union members who are supporting him so enthusiastically in 2024. “If certain parties can’t set aside the theater and negotiate in good faith, then it is incumbent on the Biden Administration to use all tools at its disposal to help mediate a resolution,” says Mr. Spear.