Boston labor leader Sean O’Brien is coming for Amazon, with a nearly 1.4 million-person army of Teamsters at his side.

O’Brien, who leads Charlestown-based Teamsters Local 25, learned on Thursday night that he won a race to be the general president of the International Brotherhood of Teamsters by a roughly two-to-one margin, of the nearly 175,000 votes cast, over opponent Steve Vairma. O’Brien will succeed longtime Teamsters chief Jim Hoffa, who endorsed Vairma.

Labor experts say the vote shows members want an attitude shift at the top of the union, which is involved in an array of industries across the United States and Canada and is best known for championing truck drivers and warehouse workers.

“It sends a message . . . that the union membership wants more aggressive leadership both in the context of contract negotiations and in the context of union organization [drives],’’ said Steve Striffler, director of UMass Boston’s Labor Resource Center. “Members recognize that the future of the union depends on going after companies like Amazon.’’

O’Brien, 49, said the first item on his agenda will be to unite the Teamsters. But organizing workers at Amazon, which has notoriously rebuffed unionization efforts, is also a top priority. (Amazon argues its workers are better served when they don’t have unions in the middle between them and management.)

O’Brien’s victory against Vairma is the culmination of a three-year political campaign that involved crisscrossing the continent. O’Brien has also been busy in Greater Boston, fighting back against Amazon’s growing reach in the area. Local 25 has persuaded numerous city councils to adopt resolutions that call on Amazon to adopt high employment standards at any local delivery or warehouse operations. The Boston City Council was the first to approve one of these resolutions, which, among other things, said Local 25 should be consulted before an Amazon warehouse opens within the city limits. Since then, similar measures were passed in a dozen other cities, most recently in Lowell last month.

“Now, as a result of this election, we’re going to be in a better position where we can use our influence to do that nationwide,’’ O’Brien said.

Some Teamsters members have begun to try to organize Amazon facilities in Canada, but O’Brien is planning on a full-court press in the United States. That will include lobbying to change state and federal laws, he said, to make it easier to organize Amazon drivers who are classified as independent contractors.

“We’ve got to make sure we engage the politicians wherever they are building these distribution facilities, wherever they are anchoring down, to ensure they’re held accountable,’’ O’Brien said. “Once we’re successful in one location, I think it’s going to be a ripple effect.’’

Another piece of the Amazon strategy: securing a stronger contract with UPS, the largest Teamsters employer. O’Brien is one of many union members who thinks the current contract, which expires in 2023, conceded too much to UPS and set a bad precedent.

“They’re going to be negotiating much stronger contracts that they’ll be able to hold up and show to workers at Amazon and other big nonunion employees, and say, ‘Here’s what the Teamsters can do for you,’’’ said John Logan, a labor and employment professor at San Francisco State University. “Now that the leadership contest has been settled, you will see a gearing up in terms of the Teamsters headquarters to put more resources and more energy into this fight against Amazon.’’

The Teamsters headquarters is in Washington, so O’Brien plans to move there for the new job, although he said he’ll continue to hold on to his Local 25 role as well, for now. “It will definitely be a change in direction and a change in culture at the top,’’ Logan said. “He is one of these younger, more assertive union leaders who are unapologetic in saying they’re going to use worker power . . . to stand up against powerful corporations.’’

Jon Chesto can be reached at jon.chesto@globe.com. Follow him on Twitter @jonchesto.