For climate-change activists, there’s an upside to the novel coronavirus outbreak.

As House Majority Whip James Clyburn, South Carolina Democrat, reportedly said last week, “This is a tremendous opportunity to restructure things to fit our vision.”

Not only did China see its greenhouse-gas emissions drop as its economy shuttered, but Democrats held up Monday nearly $2 trillion in pandemic relief to squeeze in items from their climate wish list, including re-upping renewable-energy tax credits and tacking emissions mandates on the aviation and cruise industries.

Prodding the Democrats were global-warming gurus like Bill McKibben, founder of 350.org, who urged Congress to take advantage of the unprecedented opportunity to address both the coronavirus and climate “crises.”

“Congress: don’t waste this chance,” tweeted Mr. McKibben. “If you’re going to bail out corporations, in return demand that they take serious climate action. We could actually work on both crises at once!”

Republicans erupted, accusing Democrats of putting their ideological wish list ahead of the national interest by delaying critical funds to hospitals and small businesses to “dust off the Green New Deal,” as Senate Majority Leader Mitch McConnell put it.

James Taylor, senior fellow at the free-market Heartland Institute, called it “unconscionable that Democrats are holding coronavirus relief for millions of Americans hostage to completely unrelated extremist climate demands.”

“American lives and Americans’ ability to feed themselves and make rent and mortgage payments are far more important than imposing climate restrictions on airplanes and giving more handouts to the wind and solar industries,” said Mr. Taylor. “Politicians who believe unemployed Americans are less important than climate activism and wind power have no moral justification holding office.”

JunkScience’s Steve Milloy, a member of the Trump EPA transition team, said that Democrats are “more worried about not letting the coronavirus go to political waste than they are about protecting public health and rescuing the economy.”

The pandemic has threatened to throw the U.S. economy into an economic recession, if not depression, but for the climate movement, the upheaval offers a rare, possibly once-in-a-lifetime opportunity to recalibrate societal priorities on everything from travel to commuting to shopping.

In a statement updated Monday, “5 Principles for Just COVID-19 Relief and Stimulus,” hundreds of environmental groups, including 350.org, Extinction Rebellion and Greenpeace, called for achieving the recovery while simultaneously meeting the goals of the Green New Deal.

They include public investments to “rebuild our infrastructure, replace lead pipes, expand wind and solar power, build clean and affordable public transmit, weatherize our buildings … restore our wetlands and forests, expand public services that support climate resilience, and support regenerative agriculture led by family farmers.”

In a Monday editorial, the Los Angeles Times declared, “Climate change is just as real as COVID-19. Now’s the last best chance for our government to treat it that way.”

“The main imperative for the government is to keep climate policy in mind as it devises a plan to rescue the economy,” said the staff editorial, adding that “maybe accepting the reality of COVID-19 will lead the administration to recognize the reality of climate change and work with Congress to begin addressing it in meaningful ways.”

Shutting down economy to save world

Such declarations have alarmed free-market advocates worried that the global and national response will pave the way for more government control over the private sector as future Democratic administrations draw parallels between the coronavirus and global warming.

“The goals of the climate activists have advanced, given the coronavirus’ total shutdown of society,” said Climate Depot’s Marc Morano. “Climate activists know that if the U.S. government can shut down all aspects of society over a virus, it can and may someday under a different president take similar measures to fight an alleged climate crisis.”

He pointed to the shutdown of airline travel, restaurants and entertainment, saying that “many climate activists will welcome a global recession because they have been calling for ‘degrowth’ policies and ‘planned recessions’ to fight climate change.”

It’s true that the pandemic has come at a cost to the climate movement by curbing rallies, protests and other public events. At the same time, the drop in manufacturing, travel and commuting has also led to a reduction in greenhouse-gas emissions, most notably in China, the world’s largest emitter.

Satellite imaging from NASA and the European Space Agency shows a dramatic decline in nitrogen dioxide since Jan. 1, although emissions are expected to rebound as the Chinese economy picks up again.

“Well, that is, ironically, of course, the other side of this, right? It may be good for climate … because there is less trade, there’s less travel, there’s less commerce,” said former U.N. climate chief Christiana Figueres in a March 3 interview with London’s Channel 4.

If companies decide they can hold meetings via telecommuting instead of traveling, “that would be a systemic contribution to climate change,” she said.

“What is not helpful is just a help that would be here for a few months but then we jump back to bad traditions,” said Ms. Figueres.

Mr. McKibben agreed. “Our goal can’t be simply a return to the status-quo ante, because that old normal was driving a climate crisis that will eventually prove every bit as destructive as a pandemic,” he said Friday in a New Yorker op-ed.

“With just a little courage from Democratic legislators, we could actually be building a world that is safer on every front,” he concluded.