A Hoffa ally, then a foe, and soon the Teamsters president

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 O’Brien, a fourth-generation Teamster and head of a Boston local, on a course to succeed 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 — 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 Hoffa, who has headed the union for more than two decades and whose father did from 1957 to 1971. The younger Hoffa did not seek another five-year term.

In an interview, 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.

During the campaign, 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.

In an interview, Hoffa said that 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 O’Brien’s critique of the union’s efforts on Amazon was unfair.

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.