Workers at Warrior Met Coal (HCC  ) have been on strike since April of 2021, and now lawmakers are calling on the company's largest stakeholder, BlackRock (BLK  ), to encourage Warrior Met to "negotiate in good faith and agree to a reasonable contract that treats workers with dignity and respect." Senators Bernie Sanders, Elizabeth Warren, and Tammy Baldwin wrote a joint letter to BlackRock's chief executive, Larry Fink, calling on him to act.

"As we hope you understand, the mines in Alabama run 24 hours per day. Workers can face termination for missing more than four days of work," the Senators wrote in a Jan. 13 letter obtained by CNBC. "Given BlackRock's stake in the company and your position within BlackRock, we are asking you to do the right thing."

BlackRock's stake in the coal company isn't a part of an actively managed fund. Instead, more than a dozen BlackRock index funds owned Warrior Met stock at the end of 2021, accounting for a 13% stake in the company.

While the roughly 1,000 miners represented by the United Mine Workers of America union went on strike last summer, the issues that led to the walkout began in 2016. In 2016, Warrior Met purchased the mines from their previous owner, Walter Energy. Walter Energy was bankrupt when it was purchased, and the miners bore the brunt of restructuring in the form of a $6 per hour pay cut and significant cuts in health coverage and other benefits.

Since then, the company has recovered from bankruptcy and become profitable again. Now, miners want the $1.1 billion they sacrificed in wages and benefits paid back.

"Instead of providing the kind of wages and benefits that the workers need and are entitled to, what is being offered is a totally unsatisfactory contract," Sanders said. "We're saying to Mr. Fink, 'Do the morally right thing.' What we're asking for is: 'Tell the company executives to sit down and negotiate a fair contract with the union.'"

Fink isn't the first exec to be targeted by the Senators' pleas for fairness. In December, they sent a similar letter to Warren Buffett, calling on him to encourage fair negotiation with strikers at Special Metals, owned by a subsidiary of Buffett's Berkshire Hathaway (BRK.A  ), Precision Castparts.

Buffett wrote his own letter back to the Senators, saying that he would be "making no recommendation" to the CEO at Precision Castparts, adding that "he is responsible for his business." For its part, Precision released a statement saying that it will "continue to bargain in good faith... to reach a fair agreement."

Both Buffett and Fink receive letters like this frequently, according to financial experts. BlackRock manages more than $9 trillion in investments, making Fink one of the most powerful investors in the world. If he wished to do so, he could use that power to influence or destroy any executives or company managers who disagree with him.

In the past, Fink has used that power to fight climate change, urging executives to cut carbon emissions and their environmental impact. Last year, Fink called on companies to publicly state how they would be adjusting to become "compatible with a net-zero economy." Fink said that BlackRock itself would be adjusting its investment process, including changing how non-passive funds can get rid of non-climate compatible investments.