In the summer of 1954, during hearings prompted by his investigation of Communism in the U.S. Army, Wisconsin Sen. Joe McCarthy hit his professional nadir. Speaking to Joseph Welch, chief counsel for the Army, McCarthy accused a young lawyer who worked in Welch’s law firm of having been “a member of an organization which is named, oh, years and years ago, as the legal bulwark of the Communist Party.”

Welch was stunned. “Until this moment, Senator, I think I never really gauged your cruelty, or your recklessness,” he responded. When McCarthy pressed his point, Welch asked: “Have you no sense of decency, sir, at long last? Have you left no sense of decency?”

That question became the epitaph of McCarthyism, the senator’s campaign “to dislodge the traitors from every place where they’ve been sent to do their traitorous work.” The U.S. public, or most of it, remembers McCarthy as an unhinged zealot.

But his concerns about Communism were far from groundless. Many Communists had in fact infiltrated the U.S. government. He denied it for all of his life, but Alger Hiss was indeed a spy for the Soviet Union, just as McCarthy insisted, and he did work with journalist and former Communist Whittaker Chambers, who recruited him.

Hiss was hardly alone. As Kati Marton shows in her forthcoming book on Noah Field, a Soviet spy, there was an active Communist network in the U.S. in the 1930s and 1940s.

By the time McCarthy rose to prominence, the threat was diminished — but not eliminated. (The riveting television show “The Americans” accurately depicts Soviet spying as continuing into the 1980s.) And for a period, McCarthyism could claim a degree of bipartisanship. Robert Kennedy, for example, was a fierce anticommunist who worked on McCarthy’s staff, and stayed loyal to him until his death in 1957.

The Soviet Union was an exceedingly serious danger to the U.S., and McCarthy was correct to draw attention to the threat of infiltration. But McCarthyism was nonetheless evil, because it did two things: It damaged countless innocent people, who were swept up in the scare, and it divided good Americans from one another, producing an atmosphere of distrust and suspicion with the claim that so many of our fellow citizens were not only wrong but also disloyal.

Because the Communist threat was real, it did make sense for the executive branch and Congress to call for careful security checks of federal employees and to take allegations of disloyalty seriously. In 1947, President Harry Truman issued a controversial executive order to accomplish both goals, and many of its provisions were reasonable. But McCarthyism went wrong by making reckless allegations and by treating people with left-wing political views as dangerous and fundamentally un-American. 

Which brings us to Donald Trump. No one should deny that Islamic terrorists want to kill Americans, and Trump is right to emphasize the need for careful screening procedures to keep Americans safe. But by branding his political opponents as in league with terrorists, and in calling for a new kind of Cold War, Trump is engaging in a form of 21st-century McCarthyism. In some ways, he’s outdoing McCarthy.

The most alarming line in Trump’s national security speech this week has received far too little attention. It wasn’t his claim that we should admit only those people who “share our values.” Nor was it his vague proposal for a new “immigration screening test.” The most alarming line was his identification of “the common thread linking the major Islamic terrorist attacks that have recently occurred on our soil,” which turns out to be “that they have involved immigrants or the children of immigrants.”

Really? Is that the common thread?

As of 2014, 42.4 million immigrants lived in the United States. They have had about 37.6 million children. Trump is speaking, then, of a class of about 81 million people, one-quarter of the nation’s entire population. Trump contends that 380 foreign-born individuals were charged with terrorism or terrorist-related offenses between Sept. 11 and 2014 — which is 0.0005 percent of the group that supposedly provides “the common thread.”

In view of that figure, there is no reason to feel threatened by immigrants and the children of immigrants as a class — far less reason, in fact, than McCarthy had to feel threatened by members of the Communist Party of the United States of America, who reached a high of 85,000 people in 1942. Of course terrorism is a serious danger, but in the McCarthy period, there was a genuine risk of nuclear war with the Soviet Union.

If we’re looking for a common thread to connect recent terrorist attacks, we could do a lot better than to single out a group of 81 million people. And if we begin to link immigrants and their children with terrorists, we will do enduring violence to American values.

In his way, McCarthy was a patriot, but because of his zealotry, he produced his own reign of terror. Bob Dylan got it right:

Well, I wus lookin’ everywhere for them gol-darned Reds

I got up in the mornin’ ’n’ looked under my bed

Looked in the sink, behind the door

Looked in the glove compartment of my car

Couldn’t find ’em ...

I wus lookin’ high an’ low for them Reds everywhere

I wus lookin’ in the sink an’ underneath the chair

I looked way up my chimney hole

I even looked deep down inside my toilet bowl

They got away ...

Trump is more performance artist than zealot. But he’s finding enemies everywhere, whether they are judges of Mexican ancestry, parents of those killed in war, the current president, or children of immigrants. Whether or not he has a sense of decency, he is in grave danger of losing it.

By Cass R. Sunstein

Cass R. Sunstein is a Bloomberg View columnist. He is the author of “The World According to Star Wars” and a co-author of “Nudge: Improving Decisions About Health, Wealth and Happiness.” — Ed.