Rep. Alexandria Ocasio-Cortez (D-N.Y.) and Sen. Edward J. Markey (D-Mass.) hold a news conference for their proposed Green New Deal to achieve net-zero greenhouse gas emissions in 10 years, at the Capitol on Feb. 7. (Jonathan Ernst/Reuters)

The AFL-CIO, the national arm for U.S. labor unions, offered a critical assessment of the Green New Deal, warning that the ambitious plan to combat climate change could adversely affect U.S. workers.

In a letter last week to Sen. Edward J. Markey (D-Mass.) and Rep. Alexandria Ocasio-Cortez (D-N.Y.), the lawmakers who introduced a resolution last month detailing the key components of their plan, members of the AFL-CIO’s Energy Committee said it could not support a proposal that did not address their concerns.

“We will not stand by and allow threats to our members’ jobs and their families’ standard of living go unanswered,” wrote Cecil Roberts, president of the United Mine Workers of America, and Lonnie Stephenson, president of the International Brotherhood of Electrical Workers.

The Green New Deal resolution, as proposed by Markey and Ocasio-Cortez, calls for the federal government to achieve net-zero greenhouse gas emissions with a “fair and just transition” for all communities and workers, including by creating millions of high-wage jobs, health care and housing for all, a sustainable environment and enormous infrastructure investments.

The proposal would make sweeping changes and expand the government’s reach into the economy, and it almost certainly would require tax increases or large-scale deficit spending.

It entered the national conversation when Ocasio-Cortez adopted it as her calling card. The proposal marries climate change and income inequality as one all-encompassing issue.

Support for the Green New Deal has become a benchmark for Democrats running for president.

But the AFL-CIO throwing water on the plan complicates matters for Democrats who rely on labor support. Without the backing from unions or the business community, it will be a hard sell for Democrats to get it beyond grass-roots support.

In their letter to Markey and Ocasio-Cortez, Roberts and Stephenson called the Green New Deal “not achievable or realistic.” They urged the lawmakers to include labor in conversations related to climate change, but they said such work shouldn’t impinge on other priorities such as infrastructure.

Sen. John Barrasso (R-Wyo.) tweeted the letter and added, “I agree with the AFL-CIO.”

Markey fired back on Twitter: “We will continue to work and partner w/ @AFLCIO, who is right to say that ‘doing nothing is not an option.’ But until Republicans say that climate change is real, caused by humans, and demands action now, the only people they are in agreement with are Big Oil and the Koch brothers.”

In the fall, the top scientific body studying climate change found that the world had to take “unprecedented” steps to reduce carbon levels, with the globe on pace to warm by 1.5 degrees Celsius (2.7 degrees Fahrenheit) over preindustrial levels.

The Trump administration has not proposed a comprehensive agenda for addressing climate change. It has dismantled some initiatives supported by the past administration to check the growth of greenhouse gases.

President Trump has repeatedly questioned the scientific consensus that global warming is occurring. Just Tuesday morning, Trump tweeted a quote from “Fox and Friends,” where a guest said: “The whole climate crisis is not only Fake News, it’s Fake Science. There is no climate crisis, there’s weather and climate all around the world, and in fact carbon dioxide is the main building block of all life.” Trump added: “Wow!”

The Green New Deal has become a favorite foil for Trump and congressional Republicans. Trump mocked the plan in a speech to conservatives last week, pretending to ask his wife to check the wind to determine whether they could watch television.

Senate Majority Leader Mitch McConnell (R-Ky.) has said he wants to bring the proposal to a vote to force Democrats to take a stand on it.

Rep. Peter T. King (R-N.Y.) said Tuesday the Green New Deal risks alienating labor groups, giving Republicans an opportunity with voters who side with conservatives on issues such as gun control and abortion. Exit polling from the 2016 presidential election showed a sharp decline for Democrats in support among union households.

“If Republicans play it smart and stop antagonizing labor, there’s a real opening for us,” King said.

Co-chairs of the Congressional Progressive Caucus, Reps. Mark Pocan (D-Wis.) and Pramila Jayapal (D-Wash.), acknowledged during a news conference Tuesday that labor groups have some concerns with the Green New Deal.

“Anything we move forward on, we have to be recognizing that people could lose jobs,” Pocan said.

AFL-CIO President Richard Trumka told reporters on Capitol Hill last week that labor was not consulted on the Green New Deal before it was released.

“Look, we need to address the environment. We need to do it quickly,” he said. “But we need to do it in a way that doesn’t put these communities behind, and leave segments of the economy behind. So we’ll be working to make sure that we do two things: that by fixing one thing we don’t create a problem somewhere else.”

There has long been tension between the environmental and labor movements, two major parts of the broader Democratic coalition, over worries that rules meant to curb pollution can lead to job losses in regulated industries with high-quality, good-paying positions.

The crafters of the Green New Deal sought to smooth over those concerns by incorporating into their proposal a “fair and just transition for all communities and workers” as the United States seeks to drive down climate-warming emissions from the electricity, transportation and agriculture sectors.

The resolution called for any economic transition to create “high-quality union jobs” and guarantee “wage and benefit parity for workers affected by the transition.”

Robert Hockett, a law professor at Cornell University who advised Ocasio-Cortez on the Green New Deal, argued the apprehension is misplaced because new environmental protections can lead to job growth elsewhere.

“They are probably objecting prematurely,” Hockett said. “It has become customary to think of these as separate problems.”

Yet even before Markey and Ocasio-Cortez released their Green New Deal resolution, some heavy-industry unions were already posturing against it.

Seven unions representing ironworkers, plumbers, electrical workers, boilermakers, sheet metal workers, transportation communication workers and coal miners began late last year sending a white paper to congressional offices expressing “grave concerns about unrealistic solutions such as those advocated in the ‘Green New Deal.’ ”

Instead, the unions said a cap-and-trade proposal such as the one Democrats under President Barack Obama tried and failed to pass in 2009 was a better “starting point” for new legislation.

Markey, then a member of the House, was a lead sponsor of that bill.

John Risch, who worked as a locomotive engineer for 30 years before becoming the national legislative director at the transportation division of one of the unions, the International Association of Sheet Metal, Air, Rail, and Transportation Workers, worried that any promise of a “just transition” for his members hauling coal and oil by train would end up being empty.

“We are not knuckle-draggers,” Risch said. “We’re concerned about climate change. We want to do something positive. But there are a lot of jobs on the lines.”

At least one of the main Green New Deal sponsors is recognizing — and trying to heal — the rift between environmental and labor groups over it.

Last week, staffers working for Markey met with Phil Smith, the head of communications and government affairs for the United Mine Workers of America, after the senator’s office reached out to the nation’s most prominent coal-mining union.

Smith called his meeting “a good first step.”

Still, he called the Green New Deal’s ambitions to meet all of the nation’s electric power needs with “clean, renewable, and zero-emission energy sources” within a decade a nonstarter, with coal still accounting for more than a quarter of the country’s electricity generation.