Hillary Clinton continues to vow that she’ll undo the Supreme Court’s decision in the 2010 Citizens United case, promising to introduce a constitutional amendment restricting corporate campaign activities if elected president.

This would set a dangerous course, eroding the First Amendment guarantee of freedom of expression.

Clinton and other progressives argue that the 5-4 Supreme Court ruling in Citizens United v. Federal Election Commission was a decision by the court to allow “big money” to influence elections by giving corporations, unions and other groups the same political speech rights as individuals under the First Amendment. Clinton has even suggested that the court used the case to thwart her previous presidential bid.

It’s one thing to criticize Citizens United and hope a different court would overrule the decision; the case is controversial, and the court has overruled its own opinions dozens of times in its history.

It is another thing, however, to open Pandora’s box by passing a formal constitutional amendment creating a specific limit on free speech.

Clinton’s focus on going the amendment route is among a growing and disturbing number of instances in which certain groups of people believe that certain other parties, holding views with which they disagree, are such a threat to society that they should be shut down.

Several left-leaning state attorneys general, for example, are trying to use a 1970 anti-racketeering statute—the Racketeer Influenced and Corrupt Organizations Act, commonly known as RICO—to silence so-called “climate change deniers,” including energy companies, think tanks, scientists and skeptical media organizations, like the conservative magazine National Review.

The theory underpinning the free speech assault is that these and other well-financed organizations have coordinated efforts in a conspiracy to commit intellectual fraud against the public in order to protect their financial and political interests.

The history of free speech is replete with individuals and groups pursuing their own interests, whether financial or philosophical, in the marketplace of ideas. Think the Rev. Martin Luther King Jr. and the civil rights movement; Samuel Gompers and the labor movement; Jack Welch and General Electric.

Such pursuit can be productive so long as countervailing forces are available and willing to check and criticize what they claim, leaving the ultimate determination of truth and virtue up to We the People.

Fortunately, such checking and counter-argument have been alive and well thus far.

Allowing this to change, as Clinton proposes, would give one entity—the government—the power to decide the truth for the rest of us.

An obvious slippery slope comes with this move and nothing would prevent this type of precedent from being used against the other side when a new governing coalition comes to power.

Meanwhile, a bigger question looms: Why aren’t the mainstream media defending the First Amendment, at least a vigilantly as they defend other rights?

As John Stuart Mill maintained in “On Liberty,” even ideas that we believe are 100 percent true need to be challenged in order to keep them vital and open to principled revision. Arguments are always made more credible by having to answer to critics.

In the United States, we don’t silence our critics and those with whom we disagree. We fight them with facts and ideas. The heavy hands of government stay out of the fray.