Critic of Teamsters leader claims victory in race to succeed him.

The head of a Boston local who urged a more assertive stand toward employers like the United Parcel Service — and an aggressive drive to organize workers at Amazon — declared victory Thursday in his bid to lead the International Brotherhood of Teamsters.

If the result is confirmed, the victory by Sean O’Brien, an international vice president of the Teamsters, would put a new imprint on the nearly 1.4 million-member union after more than two decades of leadership by James P. Hoffa, who did not seek another five-year term.

The outcome appears to reflect frustration over the union’s most recent contract with UPS and a growing dissatisfaction with the tenure of Mr. Hoffa, whose father ran the union from 1957 to 1971.

With about 90 percent of the ballots tallied, Mr. O’Brien had more than two-thirds of the vote in his race against Steve Vairma, a fellow international vice president who had been endorsed by Mr. Hoffa. The election was conducted by mail-in ballots that were due Monday.

Mr. O’Brien, 49, railed against the contract that the union negotiated with UPS — where more than 300,000 Teamsters work — 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 Mr. Hoffa’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.