Free speech has recently been losing its battle with political correctness. Events on campuses including Yale and the University of Missouri are Exhibit A. Less noticed but equally ominous is a campaign by environmental activists and their politician allies to use criminal and civil laws to punish fossil fuel companies for expressing doubts about the inevitability of catastrophic climate change.

Sen. Sheldon Whitehouse, D– R.I., was already calling for RICO charges when New York Attorney General Eric Schneiderman upped the ante in November with a subpoena demanding that Exxon Mobil produce decades of records about its approach to climate change. The damage Schneiderman and the rest of the Green Guards are doing to the First Amendment may well come back to haunt them.

Ostensibly, Schneiderman is investigating whether Exxon Mobil was aware of but failed to adequately warn investors about the possible risks posed to the oil industry by climate change. The truth, however, is that Exxon has explicitly disclosed those risks in its financial filings and has published more than fifty scientific papers on the evidence for and against extreme climate change. "[T]he accusation isn’t really that Exxon misled the public," notes Holman Jenkins in the Wall Street Journal. "It’s that Exxon refused to sign up for [the left's] vision of climate doom that would justify large and immediate costs to reduce fossil fuel use."

While Schneiderman is entitled to champion a vision of climate doom, he is not entitled to use the laws against securities fraud to silence those with a different vision. For one thing, securities laws are aimed at the disclosure of facts that would otherwise be unknown to investors, not at societal threats that are part of the public debate. There is little doubt that nuclear war would have a catastrophic effect on corporate profits, but companies are not obligated to discuss those risks in their financial filings.

Moreover, debate about matters of public concern, such as climate change, are at the core of the speech protected by the First Amendment. Last year, the U.S Court of Appeals for the D.C. Circuit warned against exactly what Schneiderman is attempting—allowing the government to "regulate otherwise protected speech using the guise of securities laws."

Despite the "corporations aren't people" mantra, the Supreme Court has consistently said that the speech of corporations—from tobacco companies to the New York Times—is protected by the First Amendment whether it involves commercial information or political issues. Where, as in New York, the government tries to suppress speech because it disagrees with the content, the courts impose the strictest First Amendment scrutiny.

Accordingly, any indictment of Exxon would likely be dismissed.  But winning in court is not the point of the New York investigation. Instead, it is akin to a SLAPP suit (a strategic lawsuit against public participation), which by definition, is aimed not at a legal victory but at silencing one's opponents in a public debate through intimidation and the burden of legal costs. Many states have enacted anti-SLAPP suit laws to protect free speech, which is under a heightened threat in Schneiderman's investigation because it is the government rather than a private party doing the intimidating.

Sadly, Schneiderman, Whitehouse, and the others behind this campaign are unlikely to be deterred by the law or the facts. But perhaps they can be dissuaded with a reminder that their precedent could well come back to bite them and their sacred cows.

Consider, for example, the green lobby's decades-long -- but now discredited -- warning that the world would soon run out of fossil fuels. If any company profiting from the resulting push for alternative energy had an inkling that the scarcity scenario would not pan out but failed to disclose the damage that would do to its bottom line, the company could face charges under Schneiderman's vision of securities law.

And what about the groundless claim that building the Keystone Pipeline would increase carbon emissions? Perhaps a red state attorney general should launch an investigation into which green energy companies selfishly encouraged that lie without disclosing that science and logic argued against it.

Conservative attorneys general need not limit their investigation of securities violations to misinformation about environmental issues. Consider the corporations advocating for immigration reform while arguing that it won't cost the jobs of American citizens. Let's hope those companies are explicitly warning their investors that business plans predicated on hiring immigrants will be endangered if evidence emerges that immigration reform hurts American workers.

Finally, many companies are planning to hire increasing numbers of female science graduates based on the popular theory that, as discrimination abates, half of all science degrees will be earned by women.  What if corporate executives suspect there are real differences in the scientific ability or interests of men and women but hide their doubts from investors? Should these gender-difference deniers go to jail?

If Schneiderman and his green comrades succeed, their model for silencing critics will surely be replicated by attorneys general and trial lawyers on the left and right. Progressives who don't want to go down that road should reconsider a tried-and-true approach too often eschewed by today's climate activists and student protestors. They should engage rather than silencing those with whom they disagree, trusting that if their progressive ideas are superior, those ideas will need no investigations, lawsuits or witch hunts to prevail.

Curt Levey, director of legal affairs at the FreedomWorks Foundation, is a constitutional law attorney  and veteran of Supreme Court confirmation battles. He can be reached on Twitter at @Curt_Levey.