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

The U.A.W. gained one vote to unionize a Volkswagen manufacturing facility 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 huge nonunion rivals within the supply enterprise, Amazon and FedEx.

Polling exhibits that public assist for unions is the very best it has been in a long time. However labor consultants mentioned structural forces would make it arduous for labor teams to extend their membership, which is the bottom it has been as a share of the entire work power in a long time. Unions additionally face stiff opposition from many employers and conservative political leaders.

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

Some labor consultants 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 power 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 achieve 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 realize one other 1,500. The Teamsters are additionally pursuing a authorized problem in opposition to Amazon that might permit them to realize 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 method 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 enhance pay whereas UPS’s union workers 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 workers belong to the Teamsters. A lot of them see FedEx and Amazon drivers on their routes and speak about pay, advantages and dealing circumstances.

“We make way 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 workers, over half the corporate’s unionized work power, a 26 % increase, to no less than $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 associates who’ve full-time driving jobs at Amazon make round $19 an hour, she mentioned.

As huge because the raises at UPS have been, they didn’t enhance pay by much more than inflation. The highest wage price instantly after the most recent deal, $44.25 per hour, was 22 % increased 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 couple 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 broadly mentioned on-line, spawning memes of UPS drivers arriving at prospects’ doorways in designer clothes.

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

Below 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 earnings allowed him to spend extra on his kids’s training.

Below the Teamsters’ take care of UPS, the highest hourly wage will enhance to $49 by the top of the five-year contract. Amazon mentioned in January that the common pay of employees at its supply contractors was $20.50 in the US. FedEx declined to offer a mean wage price 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 crammed 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 couple of dozen folks at every contractor may very well be time consuming and dear.

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 observe 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 positive ruling could be “a large deal” and an “inspiration for 1000’s 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 workers, due to the unionization effort. An Amazon spokeswoman, Mary Kate Paradis, disputed that.

Mr. Ervin mentioned the minimal pay for his employees beneath the Amazon contract was $19.75. “In case you are asking folks to make this a profession, it’s best to have higher work circumstances and pay the drivers extra,” mentioned Mr. Ervin, a 26-year Air Drive 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 a great 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, applications and providers for Amazon D.S.P.s and their drivers,” Ms. Paradis mentioned in an announcement.

Labor teams have made some positive factors 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 workers of its Specific enterprise come beneath 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 beneath the Nationwide Labor Relations Act, which governs employees at UPS and automakers.

Even so, some FedEx workers do belong to a union. Almost 6,000 pilots at FedEx Specific 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 throughout 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 a further $1.4 billion in labor prices in its 2022 fiscal 12 months.