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.

In 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.

But the union relieved Mr. O’Brien of his position several months later, after he had sought to include critics of Mr. Hoffa on the bargaining team, including the head of a large Louisville local who had narrowly lost the Teamsters presidency to Mr. Hoffa in the previous election despite being considered a long shot.

“I got tremendous pushback,” Mr. O’Brien said in the interview. “I wouldn’t step away from my goal of getting the right people at the table.”

Mr. Hoffa said he didn’t think it would be productive to have the Louisville leader on the team. “I didn’t want to get rid of Sean O’Brien,” he said. “Sean O’Brien was insistent.”

Two years later, Mr. O’Brien appeared at the convention of Teamsters for a Democratic Union and discussed his support for many initiatives long backed by the group, like ending a rule that required a two-thirds vote to reject a contract when fewer than half of eligible members cast ballots.

The union approved its 2018 UPS contract under the two-thirds rule even though it was opposed by a majority of the members who cast a vote.

Mr. Hoffa said that his hands were tied by the rule but that it also served a purpose: “We’re going to see how they’re going to be able to ratify contracts without the two-thirds rule,” he said. “It’s going to be certainly challenging to him.”

Ken Paff, a longtime leader of Teamsters for a Democratic Union, said Mr. O’Brien built credibility with the group by pushing for these reforms at the Teamsters convention this year, where many of them were adopted, including abolition of the two-thirds rule.

“T.D.U. could have never won on our own,” Mr. Paff said. “We’ve put them forward in the past and got creamed, but the O’Brien team backed them.” That team included Fred Zuckerman, the Teamsters leader from Louisville, Ky., who ran against Mr. Hoffa in 2016 and will now be the union’s No. 2 official, its secretary-treasurer.

Mr. Vairma, the Hoffa-backed candidate for president, supported some of the reform measures as well, including ending the two-thirds rule, and seemed to try to seize the reformist mantle himself at times during the campaign.

He portrayed a vote for his slate as a vote to diversify the union; his candidate for secretary-treasurer, Ron Herrera, a vice president, is one of the few Hispanic officials to have served in the union’s top ranks. He also tried to implicate Mr. O’Brien in the union’s slow-footed approach to Amazon. “Sean, you sat on the executive board, and I didn’t see you doing anything during your past nine years of trying to project a proactive program with Amazon,” Mr. Vairma said at one debate.

The two candidates agreed on several issues: that self-driving trucks represent a potential danger to the public and their members; that the union must fight employers’ efforts to improperly classify workers as independent contractors; and that Covid-19 vaccine mandates should not be imposed by employers without first bargaining with unions.

But the differences became apparent during sparring on the UPS contract, which Mr. Vairma accused Mr. O’Brien of “demonizing,” and in their overall posture toward employers.

Mr. Vairma warned that Mr. O’Brien was reckless, while Mr. O’Brien criticized his opponent for being overly timid. “Steve, you already conceded that in your 25-year career, you only struck six times, so UPS knows you’re not going to strike,” he said.