Democratic presidential hopefuls clambered to showcase their labor credentials this week in a bid to wow members of an influential union whose endorsement could offer a critical boost in their party’s 2020 nominating contest.

Over the course of two days at the International Association of Machinists and Aerospace Workers (IAM) legislative conference, seven contenders recalled personal ties to organized labor, railed against international trade agreements and decried right-to-work laws that they said had gutted organized labor protections across the country.

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“I stand here today as a proud, proud, proud child of unions,” Sen. Amy Klobuchar Amy KlobucharSocial media platforms put muscle into National Voter Registration Day Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (D-Minn.) said Tuesday, recalling how her mother, a teacher, had left Wisconsin for Minnesota because of the strength of teachers’ unions there.

And Sen. 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 (D-N.J.) recounted how his grandfather became a Democrat after beginning work on an assembly line during the Great Depression and joining a labor union.

“He joined a union. And that union stood up, not just for themselves — they were fighting for America, for a beloved community,” Booker said.

The appearances underscore just how eager Democrats are to court members of major trade unions at a time when many labor groups are reluctant to wade in early to the 2020 presidential race.

So far, only the International Association of Fire Fighters (IAFF) has backed a candidate in the Democratic primary, announcing late last month that it would endorse 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.

Among the candidates who spoke at the IAM conference were 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.), 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 (D-N.Y.), Klobuchar and Booker, as well as Reps. Tim Ryan Timothy (Tim) RyanNow's the time to make 'Social Emotional Learning' a national priority Mourners gather outside Supreme Court after passing of Ruth Bader Ginsburg Lincoln Project hits Trump for criticizing Goodyear, 'an American company' MORE (D-Ohio) and Seth Moulton Seth MoultonThe Hill's Morning Report - Presented by Facebook - Markey defeats Kennedy; Trump lauds America's enforcers in Wisconsin Moulton fends off primary challenges in Massachusetts Portland: The Pentagon should step up or pipe down MORE (D-Mass.) and former Rep. John Delaney John DelaneyCoronavirus Report: The Hill's Steve Clemons interviews Rep. Rodney Davis Eurasia Group founder Ian Bremmer says Trump right on China but wrong on WHO; CDC issues new guidance for large gatherings The Hill's Coronavirus Report: Kansas City Mayor Quinton Lucas says country needs to rethink what 'policing' means; US cases surpass 2 million with no end to pandemic in sight MORE (D-Md.).

The IAM announced earlier this week that it had reformed its presidential endorsement process to include a vote by its full membership, a change spurred by lingering frustration among many members with the group’s early endorsement of former Secretary of State 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 in the 2016 presidential primary contest. The union has nearly 600,000 active and retired members.

That endorsement, like those of many major unions, came months before voting ever began in the Democratic nominating contest — in August 2015 — prompting criticism from members and local affiliates who broke with union leaders to support Sanders.

The goal of the new endorsement process, IAM leaders say, is to increase transparency and make candidates compete out in the open for the union’s support. An official endorsement isn’t expected until after voting begins next year.

“It might be easier to do it the old way. We could sit down with the candidates and negotiate in a private meeting, asking them to make promises they may or may not keep,” Bob Martinez, the machinist union’s international president, wrote in a letter to members this week.

“Or we could really make them work for our endorsement.”

Each candidate that took the stage on Tuesday and Wednesday appeared intent on doing just that.

Sanders touted that he had “walked on more picket lines all over this country than I can remember,” all the while vowing to put an “immediate moratorium” on cuts to pension benefits overseen by the federal government if elected next year, a pledge that drew lingering applause from conferencegoers.

Gillibrand boasted that her home state of New York is “one of the most unionized states in the country.”

And Ryan, who has tied his presidential bid in part to a pledge to revive the country’s industrial hubs, cast himself on Wednesday as the candidate that understood blue-collar America — “a kid from steel country” who would lead a working-class takeover of a corrupt political system.

“I’m just a kid from steel country; an old high school quarterback from northeast Ohio — Friday night lights,” Ryan said. “You know what? It’s about time we have a quarterback in the White House in the United States of America.”

Democrats see the support of union workers as crucial if they hope to win in the Midwestern swing states that helped propel President Trump Donald John TrumpBiden 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 Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE to victory in 2016.

While union leaders have long backed Democrats, many rank-and-file members broke for Trump in 2016, drawn to the real estate mogul’s promises to leave or renegotiate international trade deals and attract manufacturing jobs to the U.S.

As they took the stage at the IAM conference on Tuesday and Wednesday, the candidates sought to counter that message.

While working-class people had suffered from free trade agreements, labor outsourcing and stagnant wages, several candidates argued, Trump had only made the situation more dire.

“Thousands of American jobs have been outsourced since President Trump was elected president. Thousands of workers’ livelihoods have been devastated,” Gillibrand said, adding that the president “is not on your side.”