Sean O’Brien scored a decisive victory among union members after criticizing the current leadership as too timid in UPS talks and Amazon organizing.Sean O’Brien was a rising star in the International Brotherhood of Teamsters in 2017 when the union’s longtime president, James P. Hoffa, effectively cast him aside.But that move appears to have set Mr. O’Brien, a fourth-generation Teamster and head of a Boston local, on a course to succeed Mr. Hoffa as the union’s president and one of the most powerful labor leaders in the country.A Teamsters vice president who urged a more assertive stand toward employers like the United Parcel Service — as well as an aggressive drive to organize workers at Amazon — Mr. O’Brien has declared victory in his bid to lead the nearly 1.4 million-member union.According to a tally reported late Thursday on an election supervisor’s website, he won about two-thirds of the votes cast in a race against the Hoffa-endorsed candidate, Steve Vairma, another vice president. He will assume the presidency in March.The result appears to reflect frustration over the most recent UPS contract and growing dissatisfaction with Mr. Hoffa, who has headed the union for more than two decades and whose father did from 1957 to 1971. The younger Mr. Hoffa did not seek another five-year term.In an interview, Mr. O’Brien said success in organizing Amazon workers — a stated goal of the Teamsters — would require the union to show the fruits of its efforts elsewhere.“We’ve got to negotiate the strongest contracts possible so that we can take it to workers at Amazon and point to it and say this is the benefit you get of being in a union,” he said.David Witwer, an expert on the Teamsters at Pennsylvania State University at Harrisburg, said it was very rare for the Teamsters to elect a president who was not an incumbent or backed by the incumbent and who was sharply critical of his predecessor, as Mr. O’Brien was of Mr. Hoffa.Since the union’s official founding in 1903, Dr. Witwer said in an email, “there have been only two national union elections that have seen an outside reformer candidate win election as president.”During the campaign, Mr. O’Brien, 49, railed against the contract that the union negotiated with UPS for allowing the company to create a category of employees who work on weekends and top out at a lower wage, among other perceived flaws.“If we’re negotiating concessionary contracts and we’re negotiating substandard agreements, why would any member, why would any person want to join the Teamsters union?” Mr. O’Brien said at a candidate forum in September in which he frequently tied his opponent to Mr. Hoffa.Mr. O’Brien has also criticized his predecessor’s approach to Amazon, which many in the labor movement regard as an existential threat. Although the union approved a resolution at its recent convention pledging to “supply all resources necessary” to unionize Amazon workers and eventually create a division overseeing that organizing, Mr. O’Brien said the efforts were too late in coming.“That plan should have been in place under our warehouse director 10 years ago,” he said in the interview, alluding to the position of warehouse division director that his opponent, Mr. Vairma, has held since 2012.The outcome appears to reflect frustration over the union’s growing dissatisfaction with the tenure of James P. Hoffa.Calla Kessler/The New York TimesIn an interview, Mr. Hoffa said the union was broke and divided when he took over and that he was leaving it “financially strong and strong in every which way.”He said he was proud of the recent UPS contract, calling it “the richest contract ever negotiated” and pointing out that it allows many full-time drivers to make nearly $40 an hour.He said Mr. O’Brien’s critique of the union’s efforts on Amazon was unfair. “No one was doing it a decade ago,” Mr. Hoffa said. “It’s more complex than just going out and organizing 20 people at a grocery store. He sounds like it’s so simple.”Mr. O’Brien did not elaborate on his own plans for organizing Amazon, saying he wanted to solicit more input from Teamsters locals, but suggested that they would include bringing political and economic pressure to bear on the company in cities and towns around the country. The union has taken part in efforts to deny Amazon a tax abatement in Indiana and to reject a delivery station in Colorado.Mr. O’Brien, who once worked as a rigger, transporting heavy equipment to construction sites, was elected president of a large Boston local in 2006. Within a few years, he appeared to be ensconced in the union’s establishment wing.In a 2013 incident that led to a 14-day unpaid suspension, Mr. O’Brien threatened members of Teamsters for a Democratic Union, a reform group, who were taking on an ally of his in Rhode Island. “They’ll never be our friends,” he said of the challengers. “They need to be punished.”Mr. O’Brien has apologized for the comments and points out that the reform advocate who led the challenge in Rhode Island, Matt Taibi, is now a supporter who ran on his slate in the recent election.The break with Mr. Hoffa came in 2017. Early that year, the longtime Teamsters president appointed Mr. O’Brien to a position whose responsibilities included overseeing the union’s contract negotiation with UPS, where more than 300,000 Teamsters now work.Understand Amazon’s Employment SystemCard 1 of 6A look inside Amazon.