Sen. Bernie Sanders (I-Vt.) landed a rare early endorsement from a national union on Monday amid a competitive fight for organized labor groups and working-class voters in the Democratic presidential race.

The United Electrical, Radio and Machine Workers of America threw its weight behind the independent from Vermont at its convention in Pittsburgh, where Sanders delivered a fiery address lambasting President Trump and vowing to champion workers with new initiatives and protections.

“It’s time that the working class got on the offensive,” said Sanders, who received loud applause from the audience. In a written statement, the union’s president, Peter Knowlton, called Sanders “unafraid to challenge Corporate America’s stranglehold on our economy.”

The organization, which represents some 35,000 workers, is only the second national union to make an endorsement in the primary. The International Association of Fire Fighters backed former vice president Joe Biden earlier this year. That union is much larger — it includes about 300,000 members in the United States and Canada.

Democratic hopefuls have been aggressively courting workers and unions by touting their support for a federal minimum wage of $15 per hour and their opposition to sweeping free trade agreements. Many Democrats hope embracing these platforms will help the party win back Wisconsin, Michigan and Pennsylvania, three states key to Trump’s 2016 win.

But major labor unions are taking their time to vet the Democratic hopefuls and appear far from settled on who to support. At the moment, no candidate has established dominance in the labor community, a dynamic that mirrors the competitive and crowded primary overall.

It also marks a shift from four years ago, when powerful unions weighed in earlier and largely lined up behind the eventual nominee, Hillary Clinton, who fended off a stronger than expected challenge from Sanders.

“I am sure we will endorse a candidate,” said Randi Weingarten, the president of the American Federation of Teachers, an influential union that endorsed Clinton in July of 2015. “The question becomes when, and I don’t know the answer to that question yet.”

Weingarten said her union might even wait until after next summer’s Democratic National Convention, where the party will officially pick its nominee.

The breadth of the Democratic field and conflicting loyalties to many candidates have complicated the endorsement process, labor officials have suggested.

“It’s both way more wide open this year, and I think a lot less likely that people are going to be getting a lot of union endorsements,” said Eddie Vale, a Democratic strategist who worked for the AFL-CIO, a coalition of dozens of unions.

While the power of unions to sway primary voters this election cycle is not yet clear, Democrats have pointed to several benefits of having the support of labor groups and their affiliated political organizations. Among other things, the groups have the resources to deploy ground organizers and run advertisements that reach voters beyond union members.

Sanders has intensified his efforts to appeal to unions and workers in recent weeks, releasing a proposal to expand their power and double union membership during his first four years in office. He has also shown solidarity with striking workers.

In addition, Sanders announced a key change to his Medicare-for-all insurance plan that would effectively give organized labor more negotiating power than other consumers. And in his speech in Pittsburgh on Monday, he took sharp aim at Trump, arguing his policies have been detrimental to the working class.

“You are not going to win a second term,” Sanders said of the president.

However, Sanders is far from alone in making a pitch to working-class voters anchored in union-friendly ideas and in arguing that Trump’s agenda has made life harder for the workers he claimed to support as a candidate in 2016.

Biden and his allies have underscored his roots in working class Scranton, Pa., and he has referred to himself as “Middle Class Joe” for at least a decade, though he has experienced an explosion of wealth in recent years. The former vice president has also stepped away from the Trans-Pacific Partnership, a sweeping international trade proposal that Barack Obama backed as president. Biden had served as the chief pitchman for the proposal before Congress.

Sen. Elizabeth Warren (D-Mass.), who has been rising in the polls, has also been eagerly seeking support from unions and workers. Warren apologized this past weekend after CBS News found that two of her campaign staffers had stayed at the Palms resort in Las Vegas. The powerful Culinary Workers union was picketing the resort, and Warren had endorsed their cause.

“I’m not quite sure how it happened,” Warren told reporters. “But two people stayed one night at a hotel that they should not have. And as soon as we found out about it we called, we apologized. And we’re not going to let it happen again.”

Most Democratic candidates have embraced positions demanded by unions, which has removed some of the urgency for the organizations to wade into the fluid race, Democrats familiar with the labor community said. It has prompted them to take more time deciding who to endorse — or whether to endorse at all.

“They are waiting to see which individual is going to put forth the agenda that is going to be supportive of working-class families,” said Justin Myers, chief executive of For Our Future, a political organization that includes an alliance of unions and is staying neutral in the Democratic primary.

By this point in the 2016 race, Sanders had won the support of National Nurses United, a 150,000-member union that spent millions of dollars on the senator’s behalf. The teachers’ union and the 650,000-member International Association of Machinists and Aerospace Workers, meanwhile, had endorsed Clinton.

While Clinton won over most prominent labor unions in the primary, Sanders’ allies worked to capitalize on the senator’s popularity with rank-and-file members. This year, more labor unions are involving their membership in their endorsement votes.

The 15,000-member National Union of Healthcare Workers, for example, is inviting candidates to a forum, after which its rank-and-file members will vote on who to support. The American Federation of Teachers has been holding town halls with candidates. Weingarten said the union plans to host more in the future.

But she offered few clues about who the union would eventually get behind.

“The race is still very, very dynamic,” she said.