After learning about the shooting of two lawmakers and their spouses on Saturday, President Trump appropriately denounced the tragedy, saying, “Such horrific violence will not be tolerated.”

This violence did not come out of nowhere. The landscape for it has built up over time, because of a variety of factors, including deeply polarized politics, social alienation, disinformation and in-your-face trolling on social media. Also among those factors, surely, is Mr. Trump’s pardoning of the Jan. 6 rioters. The unfortunate reality is that Americans of all political stripes are coming to see violence as a legitimate means of expressing dissent.

Whether attacks are directed at Democrats, Republicans or anyone else, they come at a steep cost to democracy: When threats and the risk of harm are unavoidable components of serving in elective office, many well-qualified candidates will conclude that the job is not worth that heavy price.

According to reports, a lone gunman shot two Minnesota Democratic state legislators and their spouses in their homes early Saturday morning, killing State Representative Melissa Hortman and her husband, Mark, and critically wounding State Senator John Hoffman and his wife, Yvette. In the vehicle driven by the suspect in the killings, Vance Boelter, investigators found a manifesto, which included a hit list of public officials and abortion providers. Regardless of Mr. Boelter’s political party preference, if any, his conduct has no place in a liberal democracy.

Mr. Trump, himself the target of two assassination attempts, has normalized conduct we once thought of as being confined to developing countries with unstable forms of government. On the first day of his second term, Mr. Trump pardoned more than 1,500 people charged with crimes committed at the U.S. Capitol on Jan. 6, 2021, including those who were convicted of assaulting police officers in a brutal and bloody attack. Our nation was founded on the principle that we resolve our disputes through informed debate, at the ballot box and in courts of law. Having failed to achieve their goals through legitimate means, the attackers instead used brute force to try to achieve their political objective when the stakes were at their very highest: while the transfer of presidential power was taking place.

Mr. Trump’s recent use of military troops to respond to protests in Los Angeles, despite objections from local officials, created an intimidating show of physical force that a judge found illegal. According to the court, the “unlawful militarization” of an American city “inflames tensions with protesters, threatening increased hostilities and loss of life.” (The court’s ruling was temporarily stayed by a higher court soon after it was issued.)