Pushed by angry members, unions representing actors, autoworkers and UPS employees are becoming increasingly assertive under new leadership.Shawn Fain is not a typical president of the United Automobile Workers union.Mr. Fain recently declined a symbolic handshake with the chief executives of the major Detroit automakers, a gesture that traditionally kicks off contract negotiations. He is seeking an ambitious 40 percent wage increase for rank-and-file members — in line, he says, with the pay gains of those corporate leaders over the past four years. And in a video meeting with members last week, Mr. Fain threw a list of proposals from Stellantis, the maker of Chrysler and Jeep, into a wastebasket, saying it belonged in the trash “because that’s what it is.”On one level, the circumstances that produced the union’s more aggressive leadership are idiosyncratic. Mr. Fain, who won his position in March, is the first president in the union’s history, dating back nearly 90 years, to be elected directly by its members. The change took place after a major corruption scandal engulfed two of his predecessors and several more union officials.But on another level, the forces that swept Mr. Fain into power are the same ones that have borne down on unions across a variety of industries: a feeling among members that they have spent years enduring out-of-touch leaders, meager wage growth and concession-filled labor agreements, which forced some to do similar jobs as co-workers for less pay.“We kept being told, ‘This is a good contract,’” said Shana Shaw, a U.A.W. member who has worked at a General Motors plant in Missouri since 2008. “And our members are saying, ‘It’s not a good contract!’”The long-simmering rage helps explain why, in addition to Mr. Fain, several prominent unions are now in the hands of outspoken leaders who have taken their membership to the brink of high-stakes labor stoppages — or beyond.Sean O’Brien, president of the International Brotherhood of Teamsters, has repeatedly referred to corporate leaders as a “white-collar crime syndicate” and warned that a strike of the union’s 300,000-plus United Parcel Service members appeared inevitable. (The union recently reached a tentative agreement that members are voting on.)Just after a union of more than 150,000 Hollywood actors called a strike in July, Fran Drescher, president of SAG-AFTRA, said that she was “shocked by the way the people that we have been in business with are treating us.” She added: “It is disgusting. Shame on them!”The companies, including UPS and the automakers, have indicated that they are willing to increase compensation but cannot jeopardize their long-term viability. The large Hollywood studios have offered actors pay increases but say they must be able to adapt to the decline of traditional television.Some executives have called out the unions’ more confrontational gestures. “The theatrics and personal insults will not help us reach an agreement,” Mark Stewart, a top Stellantis official, said in a letter to employees after Mr. Fain literally discarded the company’s proposals.And channeling members’ anger is not without risk: It can raise expectations and make it difficult for leaders to finalize contracts. Mr. O’Brien is facing a “vote no” campaign organized largely by UPS part-timers who argue that the union did not secure large enough raises.The populist approach is not unique to labor unions. The 2008 financial crisis and the grindingly slow recovery produced a more militant style of politics that upended established institutions around the world. The crisis helped lay the groundwork for the unexpected support of Bernie Sanders and Donald Trump in the 2016 presidential race.If anything, unions were slower to adapt to the rising anger than other institutions, largely because they were less democratic.In 2018, UPS employees voted down a labor contract negotiated by the Teamsters leadership, which created a new category of lower-paid drivers. The union’s president, James P. Hoffa, who had served in the position for nearly 20 years, used a procedural rule to impose the contract anyway.But even the change-averse labor movement could not withstand a final blow: Covid-19, and union members’ anger over their perilous working conditions as corporate profits grew at one of the fastest rates in decades.“There’s a historical memory of all the concessions they made,” said Ruth Milkman, a sociologist of labor at the Graduate Center of the City University of New York, referring to union members. “And they feel shafted. The C.E.O.s are sitting pretty with all this pandemic money that didn’t go into their pockets.”Many nonunion workers saw their wages rise rapidly thanks to a tight job market, but contracts negotiated before the pandemic often locked union members into smaller wage increases as inflation surged.Mr. O’Brien has tapped into that resentment.A vice president and ally of Mr. Hoffa in the mid-2010s, Mr. O’Brien ran to replace him in 2021, deriding his predecessor for foisting concessionary contracts onto members. He vowed to raise pay for part-timers at UPS — an unusual concern for a would-be Teamster president, even though part-timers make up a majority of the union’s members there — and secured a significant wage increase.Fran Drescher, center, president of SAG-AFTRA, came to channel her members’ anxiety over declining pay because of the rise of streaming.Jenna Schoenefeld for The New York TimesOther union leaders have followed a similar arc. In 2021, Ms. Drescher ran for president of SAG-AFTRA, the actors’ union now on strike, on the union’s moderate slate and narrowly won. But she came to channel her members’ anxieties over the rise of streaming, which has led to longer gaps in work for many actors and more limited royalties as shows are reused less often.“The streaming contracts negotiated back at the beginning of this, when certain individuals thought this would be a fad, set us up for failure,” said Linsay Rousseau, a SAG-AFTRA member who works primarily as a voice actor. She said Ms. Drescher’s outspokenness had won over even members who voted against her.In some cases, outraged rank-and-filers have taken matters into their own hands. Edward Hall, a rail worker and local union official in Tucson, said he decided to run for the presidency of the more than 25,000-member Brotherhood of Locomotive Engineers and Trainmen in early 2022. The union’s longtime president had arrived to hold a town-hall meeting about labor negotiations that had dragged on for over two years. But, Mr. Hall said, he was unable to provide frustrated members with a timetable for a deal. (Dennis Pierce, the former president, declined to comment.)Mr. Hall was elected last fall, shortly after Congress intervened to enact a labor agreement that members of several rail unions had voted down. Many workers felt the agreement did not go far enough to rein in a system of railroad operations that sought to minimize equipment and employees.“It was profitable for them,” Mr. Hall said, referring to rail carriers. “But for lack of a better way to put it, it made life on the railroad hell for regular employees.”The combination of agitated members and more assertive leaders can sometimes pry loose concessions from employers even without a strike, especially amid a worker shortage. This year, rail carriers began voluntarily addressing one of the workers’ biggest concerns: the lack of paid sick days.At UPS, Mr. O’Brien spent months preparing his members for a possible strike, even holding training sessions for strike captains and practice pickets. The pressure appeared to yield significant gains in the recent tentative agreement between the two sides, including more than $7 an hour in raises over the five years of the contract.In an interview last month, Mr. O’Brien said the Teamsters’ actions under his leadership had made the strike threat credible. “We’ve been striking since I took over,” said Mr. O’Brien, pointing to other companies where the union represents workers. David Pryzbylski, a labor lawyer at Barnes & Thornburg who represents employers, said the strident rhetoric of union leaders often reflected a genuine shift in workers’ attitudes. Still, he added, negotiations more often hinge on fundamentals like a company’s profitability and the union’s ability to disrupt operations through a strike, making it wise for employers to ignore the bluster.“A lot of times that stuff stops: They go out and say what they wanted to say, they send up a signal flare and move on,” Mr. Pryzbylski said. “If you start responding, it stays in the news cycle.”Sean O’Brien, president of the International Brotherhood of Teamsters, in white, has repeatedly referred to corporate leaders as a “white-collar crime syndicate.”Jenna Schoenefeld for The New York TimesThe full-throated demands can also backfire in economic terms. Yellow, a trucking company with 30,000 employees, declared bankruptcy several months after talks with the Teamsters broke down. The company’s chief executive said in a statement that the Teamsters’ intransigence drove Yellow out of business, though analysts note that the company showed signs of mismanagement for years.The risks may be even higher in industries under pressure to embrace a new business model.The major U.S. automakers have said that they need the ability to team up with nonunion battery manufacturers to secure additional capital and expertise. But Mr. Fain, the new U.A.W. president, has said that the failure to organize more battery workers was a major failure of his predecessors, and that battery workers must receive the same pay and working conditions that union workers enjoy at the Big Three.Many U.A.W. members say the tension between the automakers’ goals and the union’s indicates that a strike will be hard to avoid when their contract expires in mid-September. But they do not appear to be shrinking from that possibility.“We have an extremely well-oiled machine,” said Ms. Shaw, who also serves as a co-chair of the organizing committee of Unite All Workers for Democracy, a reform group within the union that assembled the slate of candidates Mr. Fain ran on. “We’ll be ready to go if happens.” More