The idea for Earth Day came to Gaylord Nelson all of a sudden one day in the middle of 1969. That summer, “teach-ins” about the Vietnam War were all the rage. It occurred to Nelson, then the junior U.S. senator from Wisconsin: How about a “teach-in” about the environment?

To attract the widest possible audience, Nelson, a Democrat, invited Representative Pete McCloskey, a Republican from California, to co-chair the event. The response was way more enthusiastic than either man had anticipated: on April 22, 1970, some twenty million Americans—a tenth of the country’s population—took to the streets. It was the largest public demonstration in U.S. history, and, as Jamie Henn, one of the founders of 350.org, has put it, it “had bite.” By the end of the year, a Republican President, Richard Nixon, had created the Environmental Protection Agency and the National Oceanic and Atmospheric Administration. This was followed in relatively short order by the passage of the Clean Air Act, the Clean Water Act, and the Endangered Species Act. All of these measures were approved with overwhelming bipartisan support.

Today, as Earth Day turns fifty, it’s hard to imagine more dolorous circumstances for the occasion. COVID-19 has forced online (or cancelled) virtually all the celebrations and protests that had been planned for the anniversary. The Trump Administration has barely even taken the day off from gutting the nation’s environmental regulations. (Last week, the Administration weakened rules governing the emission of mercury and other toxic chemicals from power plants; late last month, it weakened fuel-efficiency standards for cars and light trucks.)

Meanwhile, in Congress, environmental protection has become such a thoroughly partisan issue that across-the-aisle collaborations like Nelson and McCloskey’s are rarer than Amur leopards. Owing to this divide, environmental problems that have emerged since 1970 have simply gone unaddressed. Congress has not passed—or even really come close to passing—a single piece of legislation aimed at addressing climate change. (All the steps taken by the Obama Administration to try to curb carbon emissions were done through regulation.) Precisely at the “moment when such legislative action is most needed,” James Morton Turner, a professor at Wellesley College, and Andrew Isenberg, a professor at the University of Kansas, have written, it has become “almost politically unimaginable.”

How and why this happened is the subject of Turner and Isenberg’s recent book, “The Republican Reversal: Conservatives and the Environment from Nixon to Trump.” The two trace the G.O.P.’s turn against conservation to Ronald Reagan, who equated environmentalism with pessimism, and pessimism with a lack of patriotism.

Reagan combined a sunny faith in the future with an equally sunny indifference to facts. Running for President in 1980, he claimed that acid rain was not caused by power-plant and auto emissions, as scientists had shown, but by the eruption of Mt. St. Helens, in Washington State, earlier that year. Also during the campaign, he declared that “eighty per cent of our air pollution stems from hydrocarbons released by vegetation.” Once elected, Reagan appointed Anne Gorsuch—an inexperienced ideologue (and the mother of the future Supreme Court Justice Neil Gorsuch)—to head the E.P.A. Among her first moves was to propose slashing the agency’s budget by more than forty per cent. One staffer complained that morale was so low there was “no known scientific method to measure it.”

Since Reagan, control of the White House has, of course, swung between the parties, as has control of Congress. Throughout the swings, anti-environmentalism has only become that much more entrenched in G.O.P. politics. (McCloskey, it’s worth noting, switched his party affiliation from Republican to Democrat, in 2007, at the age of seventy-nine.) A great deal of money has changed hands to help change minds; according to the Web site Open Secrets, which tracks federal campaign contributions, the oil-and-gas industry contributed nearly twenty-four million dollars to House and Senate Republicans during the past election cycle, compared with five million to Democrats.

But, according to Turner and Isenberg, money is only part of the equation; the other part is votes. For Republican politicians, there’s no incentive to, say, back legislation to limit climate change: “Neither their corporate donors nor evangelicals nor the struggling Rust Belt workers who voted for Trump in 2016 see any advantage to it.” The situation is such that, as Aaron Huertas, who works with WeCanVote.US, recently pointed out, were Democrats inclined to pass meaningful climate legislation, they’d need to win not just the Presidency this fall but also a filibuster-proof majority in the Senate.

Can this situation be changed? Certainly the hour is late and the facts—if you happen to be concerned about such things—are stark. What the original Earth Day showed is that, when Americans are mobilized, remarkable things are possible. What the past few years have shown is that Americans can be mobilized by the most remarkable falsehoods. To say that the future of the world depends on which of these tendencies prevails is at this point, unfortunately, no exaggeration.