Former Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE is casting himself as a “union man” in his pursuit of the Democratic presidential nomination, but there’s no guarantee he’ll be able to rely on the labor movement to power his primary campaign.

Biden held his first campaign rally this week at a Teamsters banquet hall in Pittsburgh, boosted by an endorsement from the International Association of Fire Fighters (IAFF), the first major labor union to throw its support behind a candidate in the 2020 race.

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Biden has cut his image as a middle-class warrior, but union strategists say the shifting dynamics in primary politics and within the labor movement have opened the door to a free-for-all among the Democratic contenders for union support.

Some union insiders say that Biden’s union appeal may be limited to the “old guard” — the high-skilled tradesmen and tradeswomen, such as electricians, steelworkers and firefighters — at a time when the grass-roots energy is concentrated on the progressive left and a new generation of unions, often led by women or people of color who represent lower-skilled workers in the public and service sectors.

“The resurgent sectors of the labor movement aren’t rushing to Joe right now,” said one Democratic strategist. “I’m not saying they won’t eventually, but the labor movement is taking a much more measured approach to getting behind the nominee this time around."

The Democratic contenders, including Sens. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.) and Elizabeth Warren Elizabeth WarrenDimon: Wealth tax 'almost impossible to do' CNN's Don Lemon: 'Blow up the entire system' remark taken out of context Democrats shoot down talk of expanding Supreme Court MORE (Mass.), are fighting tooth and nail for union support.

Candidates are jockeying for position on picket lines, lavishing attention on the labor movement over social media and holding town hall events with local members.

The energy around the once stagnant labor movement underscores the degree to which unions have been reinvigorated with youthful liberal vitality.

Teacher and retail worker strikes have become must-stops for presidential candidates. And the service employee unions, with their diverse member rolls of low earners, have reemerged as a force in liberal politics in Democratic strongholds such as Pittsburgh.

“I haven’t seen this much energy around the labor movement in my 20 years of union work, and it really flies in the face of the stereotypical idea of what a union member looks like,” said Sara Nelson, the president of the Association of Flight Attendants.

“We still have fantastic union members who have done work traditionally done by men, and we’re not abandoning that. But you’re seeing a whole new brand of union member stepping forward,” she added.

Biden still has a deep well of goodwill built up among the labor movement stemming from his time in the White House, when he was former President Obama’s liaison to union members.

But the Democrats chasing him in the primary race are looking to cut into that support.

Chuck Rocha, a senior adviser to Sanders and a former political director for United Steelworkers, told The Hill that reaching out to the diverse coalition of low-wage union members is the “bedrock” of the campaign’s Midwest strategy.

“The fastest-growing unions in the country are the service-sector employees. They tend to be younger, they tend to be more diverse, and they tend to be more progressive,” Rocha said. “That’s a universe that we’re going to be targeting.”

Sanders, who has staffed his campaign with union veterans, showed strength with the labor movement in 2016.

After the large national organizations endorsed Hillary Clinton Hillary Diane Rodham ClintonDemocratic groups using Bloomberg money to launch M in Spanish language ads in Florida The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day More than 50 Latino faith leaders endorse Biden MORE early on, Sanders was able to peel off support from local affiliates and landed the backing of several up-and-coming union groups, such as National Nurses United (NNU), which is led by women.

But union leaders, many of whom feel burned after backing Clinton early in the 2016 primary, say they’re taking their time before endorsing a candidate this go-round. The NNU has not endorsed in the 2020 primary yet.

“We’re going through a very deliberative process,” said Kenneth Zinn, the NNU's political director. “We’re engaging with all of the candidates to see where they stand.”

Last cycle, Clinton locked down the support of more than a half-dozen of the nation’s largest unions, including the Service Employees International Union and the American Federation of State, County and Municipal Employees, which together account for 3 million members before any primary or caucus votes had been cast.

But in the general election, Clinton did not campaign at a union hall in Michigan or Pennsylvania, and she did not campaign in Wisconsin at all. Her margin of victory among union households ended up being far smaller in those states than it was for past Democratic candidates.

An internal poll by the American Federation of Labor and Congress of Industrial Organizations (AFL-CIO) conducted after the 2016 election, for instance, showed that Clinton's share of members' votes was 10 percentage points less than Obama's in 2012.

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That same poll showed that Trump outperformed his party’s 2012 nominee, Mitt Romney Willard (Mitt) Mitt RomneyBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power The Memo: Trump's strengths complicate election picture MORE, by 3 points.

In a sign of just how eager he is to hold on to the working-class voters who helped deliver him the White House in 2016, Trump lashed out this week at the leadership of the IAFF after the group backed Biden, retweeting nearly 60 accounts rebuking the endorsement and insisting that rank-and-file union members supported his reelection.

“Trump figured out a way to tap into the frustration from those who felt they’d been abandoned by the establishment and by the Democratic Party,” said Randi Weingarten, the president of the American Federation of Teachers. “They wondered where Democrats were as rural America was hollowed out and industries left their states. ... But Trump has betrayed his promises to the middle class, so Democrats have to make that case and stay focused on economic issues.”

This time around, many unions are adopting a bottom-up approach to the endorsement process, having the candidates meet with members at local affiliates to ensure the grass roots are adequately represented in the process rather than leaving the decision to board members in Washington.

The International Association of Machinists and Aerospace Workers, one of the unions that backed Clinton early on in the 2016 nominating contest, is set to roll out a new endorsement process at its legislative conference in Washington next week.

Seven Democratic presidential hopefuls, including Sens. Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (N.J.), Kirsten Gillibrand Kirsten GillibrandSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Suburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits MORE (N.Y.) and Sanders, are slated to speak at that conference.

The machinist union’s new endorsement process will hinge on a vote by the group’s members, according to two people familiar with the plans, a significant departure from its 2016 endorsement, which came after a unanimous vote by union leaders and an internal survey of 1,700 of its active members.

The American Federation of Teachers has adopted a four-step process that includes questionnaires for the candidates and getting as many of them as possible in front of rank-and-file members at local town halls so members feel they “have ownership of the process,” Weingarten said.

“The unions are going through an exhaustive process with local leaders and members and engaging with as many candidates as possible,” said Steve Rosenthal, the former political director for the AFL-CIO. “I can’t remember ever having this strong a field of pro-union candidates. Unions are popular again. It’s going to be tough for members to sort through this extraordinary field.”