Labor unions are closing ranks around Hillary Clinton’s presidential campaign. 

The Service Employees International Union (SEIU) on Tuesday became the latest major labor group to back Clinton’s bid for the White House.

“Hillary Clinton has proven she will fight, deliver and win for working families,” SEIU International President Mary Kay Henry said in a statement. 

The SEIU endorsement is a big victory for Clinton and a blow to Sen. Bernie Sanders (I-Vt.), her closest rival for the Democratic nomination, who has worked hard to court labor support while championing a populist economic platform.

The SEIU, which has about 2 million members nationwide, has been a driving force behind a nationwide campaign to raise the federal minimum wage $15 an hour. Its membership is diverse, with a strong presence in Nevada, which holds one of the earliest nominating contests.

“As President, I will be proud to stand with SEIU and fight alongside them — to defend workers’ right to organize and unions’ right to bargain collectively, to raise incomes for working people and the middle class, and to ensure that hardworking Americans can retire with dignity and security,” Clinton said in a statement.

The SEIU endorsed President Obama over Clinton in their 2008 primary battle, providing a critical boost to his campaign.

Earlier this fall, unions appeared to be staying on the sidelines while pushing for Clinton to move to the left on issues important to labor.

Some labor leaders also appeared to be withholding support in case Vice President Biden entered the race, which he declined to do.

With Clinton now on record as opposing the Trans-Pacific Partnership (TPP) — a trade deal that labor leaders have fought against, tooth and nail — and Sanders failing to gain traction in the polls, her union endorsements are beginning to pile up.

In addition to the SEIU, Clinton has won the backing of the American Federation of State, County and Municipal Employees, which represents 1.6 million public sector workers, as well as the American Federation of Teachers and the National Education Association.

Altogether, the former secretary of State has endorsements from a dozen national unions with millions of members, her campaign said.

“This is going to make life so much easier for Hillary,” said Brad Bannon, a Democratic strategist and political science professor at Suffolk University. “The SEIU’s endorsement could have a domino effect. Watch for some of the other unions to come around to Hillary sooner than later."

The biggest remaining holdout is the AFL-CIO, the nation’s largest labor federation.

The AFL-CIO typically doesn’t endorse in primaries, with the unions that make up the federation often backing candidates of their own.

Labor support is usually critical in the early-voting states and beyond, with unions typically serving as a source of campaign cash for Democrats.

Unions also have extensive get-out-the-vote operations that could make a difference in close contests.

Bannon said Clinton's SEIU endorsement could be a fatal blow to Sanders’s chances.

“Bernie isn’t gaining traction with the unions,” he said. “That’s what his campaign is all about. If that wing of the party isn’t going to support Bernie, I don’t know who will.

“The curtain is starting to close on the Sanders campaign.”

Sanders has made issues of importance to labor unions the focus of his campaign, with income inequality central to his message. He has made frequent appearances at SEIU-backed rallies to support low-wage workers.

Though the SEIU is getting behind Clinton, the Sanders campaign on Tuesday said it still expects many rank-and-file union members to vote for him.

“We are gratified that hundreds of thousands of workers are part of the growing grassroots movement supporting Bernie’s campaign to help working families by raising the minimum wage, providing healthcare for all and making college affordable,” Sanders spokesman Michael Briggs said.

Clinton’s success in winning over unions comes after a fall during which she moved decisively to the left on several issues.

Perhaps the most important shift for labor was Clinton’s decision to publicly oppose the TPP, a sweeping Pacific Rim trade deal that President Obama is trying to push through Congress.

“I appreciate the hard work that President Obama and his team put into this process and recognize the strides they made. But the bar here is very high and, based on what I have seen, I don’t believe this agreement has met it,” Clinton said on Oct. 7.

Labor unions were delighted by Clinton’s announcement. They say the deal will result in lost jobs and lower wages for American workers.

Still, unlike Sanders, Clinton has declined to endorse the $15 minimum wage, saying she wants to instead set it at $12 "and encourage other communities to go even higher."

Henry recently said the SEIU would not make supporting the $15 minimum wage a litmus test for candidates.

"What we're expecting is candidates up and down the ticket who are willing to get in the streets and champion this demand," Henry told Al Jazeera.