Final 12 months, two unions representing employees at three giant automakers and UPS negotiated new labor contracts that included massive raises and different features. Leaders of the unions — the United Car Employees and the Teamsters — hoped the wins would assist them set up employees throughout their business.

The U.A.W. received one vote to unionize a Volkswagen manufacturing unit in Tennessee final month and misplaced one this month at two Mercedes-Benz crops in Alabama. The Teamsters have made even much less progress at UPS’s massive nonunion rivals within the supply enterprise, Amazon and FedEx.

Polling exhibits that public help for unions is the best it has been in many years. However labor specialists mentioned structural forces would make it exhausting for labor teams to extend their membership, which is the bottom it has been as a proportion of the full work drive in many years. Unions additionally face stiff opposition from many employers and conservative political leaders.

The Teamsters present an instructive case examine. Most of the employees doing deliveries for Amazon and FedEx work for contractors, usually small and medium-size companies that may be exhausting to prepare. And supply employees employed immediately by FedEx in its Categorical enterprise are ruled by a labor regulation that requires unions to prepare all related employees on the firm nationally without delay — a harder normal than the one which applies to organizing staff at automakers, UPS and different employers.

Some labor specialists additionally mentioned the Teamsters had not made as forceful a push because the U.A.W. to prepare nonunion employees after securing a brand new contract with UPS.

“You didn’t have that vitality that you simply noticed with the U.A.W.’s leaders,” mentioned Jake Rosenfeld, a sociologist who research labor at Washington College in St. Louis.

Teamsters officers mentioned the UPS deal, which will increase the common annual compensation, together with advantages, of a UPS driver to $170,000 from $145,000, was serving to them acquire members. At DHL, a supply firm the place the union has lengthy had an enormous presence, the union added 1,100 members final 12 months and is pushing to achieve one other 1,500. The Teamsters are additionally pursuing a authorized problem towards Amazon that might enable them to achieve floor on the firm and its contractors.

“It’s been very useful for us to mobilize,” Sean O’Brien, the Teamsters’ president, mentioned in an interview, referring to the UPS contract. “We’ve set the usual within the business.”

However the union has additionally suffered losses. Yellow, a trucking firm that employed 24,000 Teamsters, shut down and filed for chapter safety final 12 months.

Amazon and FedEx mentioned they have been assured of their strategy to managing and compensating employees. Amazon mentioned it had made investments that bolstered pay and advantages at its supply contractors. FedEx mentioned its nonunion mannequin allowed it to shortly improve pay whereas UPS’s union staff have been sure by the phrases of five-year contracts.

“Our tradition, constructed and examined over 50 years, relies on the philosophy that if we care for our folks, they are going to ship excellent service for our prospects, which can drive enterprise outcomes for our firm,” Tracy Brightman, FedEx’s chief folks officer, mentioned in an announcement,

Round 310,000 UPS staff belong to the Teamsters. Lots of them see FedEx and Amazon drivers on their routes and speak about pay, advantages and dealing situations.

“We make rather more cash than anybody else within the business,” mentioned Essence Carlisle, a part-time package deal handler at UPS’s hub in Louisville, Ky. “I positively intend to make a profession right here.”

The UPS deal gave part-time staff, over half the corporate’s unionized work drive, a 26 % elevate, to a minimum of $21 per hour. Ms. Carlisle makes near $24 an hour and works round 20 hours every week, giving her time to run a bakery on the aspect, she mentioned. Her pals who’ve full-time driving jobs at Amazon make round $19 an hour, she mentioned.

As massive because the raises at UPS have been, they didn’t improve pay by much more than inflation. The highest wage fee instantly after the newest deal, $44.25 per hour, was 22 % larger than 5 years earlier. Over that interval, client costs rose 21 %.

And UPS usually hires new union employees into part-time jobs, which they maintain for a number of years. Because of this, some folks could also be unwilling to hunt a job on the firm.

Even so, final 12 months’s Teamsters contract was extensively mentioned on-line, spawning memes of UPS drivers arriving at prospects’ doorways in designer clothes.

“Joking or not, everyone was like, ‘Hey, man, I would like a job at UPS,’” mentioned Juan Martinez, a UPS driver in Southern California.

Beneath the brand new contract, Mr. Martinez expects to earn $110,000 to $120,000 a 12 months, relying on how a lot extra time he will get, he mentioned. He mentioned his revenue allowed him to spend extra on his kids’s training.

Beneath the Teamsters’ take care of UPS, the highest hourly wage will improve to $49 by the tip of the five-year contract. Amazon mentioned in January that the common pay of employees at its supply contractors was $20.50 in america. FedEx declined to supply a mean wage fee for its supply employees.

Regardless of UPS’s superior pay through the years, the Teamsters haven’t made many inroads at FedEx or Amazon.

The excessive turnover of supply and warehouse employees at Amazon and FedEx — the place every part-time place was on common stuffed and vacated twice final 12 months — makes it troublesome to prepare them.

One other problem is that supply employees at Amazon, and drivers who ship for FedEx Floor, are employed by contractors. Mr. Rosenfeld, the labor tutorial, mentioned attempting to prepare a number of dozen folks at every contractor might be time consuming and expensive.

Final 12 months, 84 employees at an Amazon contractor close to Los Angeles joined the Teamsters. However days earlier, Amazon terminated the contract with the operator, Battle-Examined Methods, the corporate mentioned, for failing to comply with correct security procedures, amongst different issues.

The Teamsters requested the Nationwide Labor Relations Board to rule that Amazon was a joint employer of the employees and order the corporate to reinstate the contract. The board has but to rule.

A good ruling can be “a large deal” and an “inspiration for hundreds of different employees throughout the nation,” mentioned Randy Korgan, a Teamsters official.

Johnathon Ervin, the proprietor of Battle-Examined Methods, mentioned he believed Amazon had terminated the contract, which led to job losses for all his staff, due to the unionization effort. An Amazon spokeswoman, Mary Kate Paradis, disputed that.

Mr. Ervin mentioned the minimal pay for his employees underneath the Amazon contract was $19.75. “If you’re asking folks to make this a profession, you need to have higher work situations and pay the drivers extra,” mentioned Mr. Ervin, a 26-year Air Power veteran.

Amazon didn’t reply on to that criticism. The corporate famous that its contractors, which it refers to as supply service companions, had created 279,000 driving jobs over the past 5 years.

“Serving to D.S.P.s create an excellent general working expertise is essential to us, which is why we’ve invested greater than $8 billion in state-of-the-art expertise, security options, charges, packages and companies for Amazon D.S.P.s and their drivers,” Ms. Paradis mentioned in an announcement.

Labor teams have made some features at Amazon, together with organizing employees at a Staten Island warehouse. However Amazon is difficult the election there, and that union has been embroiled in infighting.

At FedEx there may be one other potential barrier to unionization.

FedEx was based as an airline, and staff of its Categorical enterprise come underneath the Railway Labor Act, which requires unions to prepare nationally, throughout a complete firm, without delay. Union officers say it’s simpler to carry particular person votes at every firm location as allowed underneath the Nationwide Labor Relations Act, which governs employees at UPS and automakers.

Even so, some FedEx staff do belong to a union. Practically 6,000 pilots at FedEx Categorical are represented by the Air Line Pilots Association. The Teamsters are trying to organize the mechanics who work on the corporate’s planes.

FedEx mentioned its supply employees had benefited from not being in a union as a result of the corporate raised wages considerably through the home-delivery increase of 2021 and 2022, when UPS employees’ raises have been set by an settlement reached earlier than the pandemic. A FedEx spokeswoman famous that the corporate incurred an extra $1.4 billion in labor prices in its 2022 fiscal 12 months.