• AFL-CIO President Richard Trumka died Thursday at age 72.
• A former coal miner, Trumka had served as president of the 12.5-million-member labor federation since 2009.
• President Joe Biden called Trumka a close friend, while Senate Majority Leader Chuck Schumer said, "The working people of America have lost a fierce warrior at a time when we needed him most."

AFL-CIO President Richard Trumka, a former coal miner who rose to lead the 12.5-million-member labor organization, died Thursday. He was 72.

Trumka, who became leader of the nation's most powerful labor organization in 2009, died of an apparent heart attack, according to two sources who had been briefed by AFL-CIO aides.

At the time, Trumka "was doing what he loved, spending time, celebrating his grandson's birthday," AFL-CIO Secretary-Treasurer Liz Shuler said in a note to staff.

"We are heartbroken," wrote Shuler, who under the group's constitution will perform the duties of president until the AFL-CIO's Executive Council elects a successor to Trumka.

After learning of the labor leader's death, President Joe Biden called Trumka a close friend. Biden's 2020 run for the White House was endorsed by the AFL-CIO.

"The labor movement, the AFL-CIO and the nation lost a legend today," said Tim Schlittner, communications director of the federation, which is comprised of 56 union affiliates and is major force in Democratic politics.

"Rich Trumka devoted his life to working people, from his early days as president of the United Mine Workers of America to his unparalleled leadership as the voice of America's labor movement," Schlittner said.

"He was a relentless champion of workers' rights, workplace safety, worker-centered trade, democracy and so much more. He was also a devoted father, grandfather, husband, brother, coach, colleague and friend. Rich was loved and beloved."

Senate Majority Leader Chuck Schumer, D-N.Y., choked back tears as he spoke on the Senate floor about Trumka.

"I rise today with some sad, horrible news about the passing of a great friend Rich Trumka who left us this morning," Schumer said before pausing to compose himself.

"The working people of America have lost a fierce warrior at a time when we needed him most."

Trumka grew up in the coal-mining town of Nemacolin, Pennsylvania. As a college and law school student, Trumka worked as coal miner, as his father and grandfather had done.

At 33, he ran on a reform ticket for the presidency of the United Mine Workers of America and won, becoming the UMW's youngest leader in its history.

In 1995, Trumka was elected secretary-treasurer of the AFL-CIO, which had been formed 40 years earlier by merger of the American Federation of Labor and the Congress of Industrial Organizations.

Trumka more recently was a major force in Biden's selection of Marty Walsh as secretary of the Labor Department.

As Biden was assembling his Cabinet, Trumka's lobbying for the then-Boston mayor was crucial to cementing Biden's choice to nominate Walsh over Rep. Andy Levin, the Michigan Democrat who was the preferred candidate of some of the AFL-CIO's affiliated unions

Trumka was equally influential when Republicans occupied the White House.

In 2019, he persuaded several skeptical Democratic House members, including House Speaker Nancy Pelosi, D-Calif., to pass then-President Donald Trump's revised version of the North American Free Trade Agreement, known as the USMCA.

Labor unions have long criticized NAFTA, claiming it sent tens of thousands of U.S. union manufacturing jobs over the border to Mexico, where wages are lower and labor unions represent industries, and not the workers in them.

Trumka later said that while USMCA was far from perfect, it was a large step toward undoing the harm caused by NAFTA. USMCA passed the House in December 2019, with 41 Democrats voting against it.

While Trumka was influential, his rise in union politics since the 1980s coincided with a marked drop in membership in American unions during that time.

In 1983, about 20% of U.S. workers belonged to a labor union, but by 2019 that had fallen to just above 14%, according to Labor Department statistics.

But in recent years, the labor movement has gained momentum, as employees have pushed for better wages and improved working conditions across industries from fast food to aviation to large retailers such as Amazon. That push has come at the same time as corporate profits have soared.

Trumka noted that shift in momentum during his last major speech on July 27, at the virtual convention of the Texas AFL-CIO.

"My fellow union members, make no mistake about it: The labor movement in Texas is growing more powerful," Trumka said. "The anti-worker attacks have not discouraged you! The uphill climb has not stopped you. Since the pandemic hit, you've done the hard work. You've made your voices louder. And you've made your communities and state stronger."

"So it should come as no surprise that America is turning toward the values of unionism."

Sara Nelson, president of the Association of Flight Attendants-CWA, which represents some 50,000 cabin crew members at more than a dozen airlines, said she was "shocked and saddened" by Trumka's death.

"The very best way to honor Rich's legacy is to fight back stronger than ever for American workers," Nelson said.

Pennsylvania Lt. Gov. John Fetterman said Trumka's death was "truly heartbreaking."

"We lost a larger than life figure who spent a career fighting for, and defending the Union Way of Life," Fetterman, a Democrat, wrote in a tweet.

"It's left to the rest of us to pick up the slack and never stop fighting.  #UnionStrong."

New Jersey Gov. Phil Murphy ordered flags in that state flown at half-staff to mark Trumka's death.

"America's and New Jersey's working families have lost one of their most steadfast and dedicated allies," Murphy said in a statement. "Organized labor has lost one of its most powerful voices."

Correction: An earlier version misspelled AFL-CIO Secretary-Treasurer Liz Shuler's last name.

- Additional reporting by CNBC's Kevin Breuninger