Donald Trump’s rally speech in Phoenix on August 22 was full of falsehoods. He gave a revisionist and false history of his reaction to the Charlottesville violence to make himself look better, made false statements about media reporting and misled the audience over his economic achievements. The speech points to the normalization of post-truth politics, when appeals to personal beliefs and emotions wins out over objective facts. To avoid this normalization, we need to borrow the successful tactics of the environmental movement.

The rally speech represents part of a broader pattern: Of Trump’s statements fact-checked by Politifact, an astounding 49 percent are false. By comparison, his Democratic opponent in the U.S. presidential election, Hillary Clinton, has 12 percent of her fact checked statements rated false; 14 percent of Republican Speaker of the House Paul Ryan’s are.

Despite Trump’s extremely high rate of deception, many still believe him. As an example, 44 percent of those polled believed his falsehoods about Obama wiretapping Trump Tower during the 2016 election campaign. Thus, many will believe his Phoenix rally claims, despite the debunking by fact checkers. Unfortunately, 29 percent of the public, and only 12 percent of Trump supporters, trust fact checkers.

Moreover, research on debunking falsehoods shows such debunking sometimes backfires. Called the backfire effect, scientists have shown in a number of cases people believe in falsehoods even more strongly after being presented with contradictory evidence. This situation enables Trump to pollute our politics with deception, destroying trust in our democratic political system.

Political and social science research summarized in the 2003 Trust and Governance, edited by Valerie Braithwaite and Margaret Levi, shows trust is vital for healthy democracies. Citizens in a democracy have a basic expectation of their public officials being trustworthy, in their words and actions. In return, citizens comply with laws, pay taxes and cooperate with other government initiatives. By comparison to a democracy, an autocratic state bears a much higher resource burden of policing to make its citizens comply with its laws. In his 2002 work, Trust and Trustworthiness, political scientist Russell Hardin also shows the vital role of trust in creating and cultivating civil society in a democracy. When political leaders act in ways that destroy trust—as Trump is doing through misleading statements and outright lies—people will increasingly stop complying with laws, paying taxes and engaging in civil society. Trump’s actions are fatally undermining the health of our democracy.

His behavior falls within the sphere of what behavioral scientists term “tragedy of the commons,” following a famous 1968 article in Science by Garret Hardin. Hardin demonstrated that in areas where a group of people share a common resource—the commons—without any controls on the use of this resource, individual self-interest may often lead to disaster for all involved. Because each individual may well have a strong interest in using more of the common resource than is their fair share, all suffer the consequences of depletion of that resource. Environmental pollution is a clear example where the common resource of clean air and water is abused by polluters who destroy this shared resource.

Trump is abusing the commons of trust in our political environment, and he is setting a clear example for other politicians to follow through his successful tactics. West Virginia Attorney General Patrick Morrisey and Kentucky Gov. Matt Bevin are adopting the post-truth tactics of condemning media as “fake news” whenever the media report stories unfavorable to them. As an example, Bevin personally attacked a journalist who reported on Bevin’s purchase of a mansion for about a million dollars under market value from a hedge fund manager, likely a bribe in return for under-the-table political favors. Such trickle-down of post-truth politics points to its normalization within our political system, thus enabling corruption and undermining our democracy.

How do we stop this pollution of truth? The modern environmental movement has been dealing successfully with a tragedy of the commons: industrial pollution. The historical consensus is that the launch of the modern environmental movement came with the publication of Rachel Carson’s Silent Spring in 1962. This and other similar publications brought about an awakening of the public to the danger posed by environmental pollution to individual and community health, and led to the coordinated movement of activists—Republican and Democrat—fighting for the environment.

As a result, environmental problems drew much wider public attention. Consider the 1969 fire on the Cuyahoga River in Cleveland. The river has had a long history of pollution, and in June 1969 oil-covered debris caught fire, causing $100,000 worth of damage to two railroad bridges. This event drew national attention and became a major story in TIME. Cleveland’s mayor testified before Congress to urge greater attention to pollution by the federal government. Notably, the Cuyahoga River had experienced many other fires due to industrial pollution, such as one in 1952 that resulted in over $1.3 million in damage—10 times that which incurred in 1969. This much bigger and more destructive fire, however, inspired little national attention—or efforts to change the situation—as compared with the conflagration of 1969.

The marked difference in the reaction to the two fires stemmed from the launch of the modern environmental movement, combining the coordinated actions of activists to seek out and highlight these problems with heightened public attention awareness of the danger of environmental pollution. We can do the same for the pollution of truth by launching a nonpartisan pro-truth movement. Such a movement would require a coordinated group of activists holding public figures accountable for deception as well as publicly highlighting the danger that post-truth politics poses to the health of our democracy.

Whereas the 1960s required the publication of books to raise awareness and launch a movement, our contemporary digital environment provides easier tools. One example is the Pro-Truth Pledge project at ProTruthPledge.org, which allows private citizens and public figures to take a pledge committing them to 12 behaviors that research suggests are most likely to lead to a truth-oriented society. This site both offers a coordination venue for those determined to roll back the tide of lies and protect our democracy, and raises awareness of the dangers of political deception. Hundreds of private citizens across the U.S . and dozens of public figures have already taken the pledge, including household names such as Peter Singer and Steven Pinker as well as over 20 Democratic and Republican politicians.

By launching a pro-truth movement uniting people across the political divide, we can avoid the normalization of post-truth politics. Doing so will help ensure that the kind of falsehoods uttered by Trump at the Phoenix rally get a response equivalent to the 1969 fire on the Cuyahoga river, rather than the 1952 one. Whether the pro-truth movement takes off depends on how many people choose to take the pledge and join the effort to protect the health of our democracy from the pollution of truth.