WASHINGTON (Reuters) - The head of the main U.S. mine workers’ union has invited all of the Democratic presidential candidates to visit its “turf” to demonstrate that the party still represents the working class.

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The invitation could heap pressure on the nearly two dozen Democrats vying for the White House to explain how their plans to combat climate change - most of which call for an end to fossil fuels use - will impact mining jobs.

“If Democrats are to win back the vote of so many who have deserted the party in the last several elections, it is exactly these kinds of voters who you need to persuade,” United Mine Workers of America President Cecil Roberts said on Monday in a letter to the candidates that asked them to speak to miners on their “turf.”

Roberts told Reuters on Tuesday that since sending the letter, at least half of the Democrats running for president have said they were interested in UMWA’s invitation to tour underground mines, speak with miners and listen to their concerns about the future of coal.

“Those candidates that are willing to go underground will have an opportunity to talk to coal miners in the place they work and understand what they do,” he said.

President Donald Trump frequently put coal miners in the spotlight in his 2016 presidential campaign, vowing to put them back to work, in part by cutting environmental regulations ushered in by the Obama administration.

Despite this, coal-fired power plants have closed under Trump’s presidency at a faster rate than during the Obama administration and some of the biggest coal miners have declared bankruptcy. The country’s sixth-largest coal producer, Blackjewel LLC, filed for bankruptcy last week, putting 600 Wyoming miners out of work without warning.

The UMWA endorsed former President Barack Obama in 2008, when he promised to put resources into commercializing clean coal technology. But it avoided endorsements in both 2012 and 2016.

“If we decide as a nation that we are going to have a completely carbon free electrical system that would cost trillions of dollars, I am guessing that for a lot less we can figure out how to burn coal cleanly,” Roberts said.

He said he wants to see candidates address vital issues for miners, including how to preserve their pensions, protect their black lung disability coverage and health care.

Most of the Democratic presidential candidates have endorsed aggressive climate change policies that would usher in a rapid shift away from the use of coal in favor of renewable energy.

Sixteen of the 20 leading Democratic contenders, for example, have endorsed or co-sponsored the so-called Green New Deal, a non-binding congressional resolution to end the fossil-fuel economy within a decade through massive government investments in clean energy. The resolution was introduced by Senator Ed Markey and Representative Alexandria Ocasio-Cortez.

Although that platform outlines several policies aimed at strengthening unions, protecting workers’ pensions and benefits and retraining, unions like UMWA have been wary of endorsing a Green New Deal and wary of policies guaranteeing workers a “just transition.”

“We can retrain people until the cows come home here but their standard of living is going to be reduced,” Roberts said. “We want to hear how we retrain somebody to be part of the middle class.”