Mo Udall, the legendary Democratic congressman from Arizona, was brutally candid about his party’s bad habit of succumbing to intramural recriminations that became the political equivalent of a civil war in the leper colony. “When Democrats organize a firing squad, they form a circle,” Udall wisely observed.

Politics, let it be noted, is a matter of addition, not subtraction. Putting together a majority to pass legislation to aid widows and orphans or a majority to win elections requires winning converts to your side rather than hunting down and banishing heretics to the Outer Darkness. Nobody understood this principle better or practiced it more successfully than the late “liberal lion of the Senate,” Massachusetts eight-term senator Ted Kennedy.

When Kennedy died, he was universally praised for his effectiveness:

“His greatest strength as a legislator was his ability to reach across the aisle, to compromise and get important work done.”

“Kennedy represented an increasingly, and sadly, rare Washington collegiality and practicality.”

“This Democrat’s true effectiveness was in his ability to compromise with Republicans to get his initiatives enacted into law.”

Those Kennedy initiatives included, to name a few, Children’s Health Insurance Program for children of working parents who did not get health insurance from their employers, mental health parity in coverage, immigration reform, AIDS research, ending apartheid, the Americans with Disabilities Act, voting rights and special education funding. Among the Republican senators he worked closely with to write laws were Mike Enzi and Alan Simpson of Wyoming, Thad Cochran of Mississippi, Orrin Hatch of Utah, Nancy Kassebaum and Bob Dole of Kansas, John McCain of Arizona and Warren Rudman and Judd Gregg of New Hampshire. Kennedy was able to do all that by seeking common ground, by never demonizing his opponents, by never making the perfect the enemy of the good.

But now we’re in a different political era. The president of the United States regularly demonizes his political opponents, labeling Democrats as “evil.” The Democratic Party, he told a rally, is “the party of crime.” Make no mistake: More than a few Democrats have responded the same way, censuring Donald Trump in similar rhetoric.

For me to call my political opponent mistaken or misguided on a particular controversy is acceptable and does not preclude her and me working together constructively in the future on a different issue. But when I call you, or you call me, “evil” or “immoral” or “irredeemable,” we have foreclosed possibility of future collaboration. Who in good conscience can collaborate with someone who is “evil,” “immoral” and worse?

So now we have Democratic presidential hopefuls Bernie Sanders, Cory Booker, Kamala Harris, Elizabeth Warren and Bill de Blasio, all of whom have criticized — in some cases, condemned — Joe Biden for speaking positively of the Senate in which he served alongside Southern segregationist colleagues Herman Talmadge of Georgia and Jim Eastland of Mississippi. Biden refuses to demonize those who disagreed with him, because he knows that demonizing your opponent makes almost inevitable a response in kind and debases the public debate. To show decency to those with whom you disagree is not a weakness but rather a strength. It helps create a climate of trust and respect in which compromise and consensus can exist.

On this one — and it’s a big one — Joe Biden is right, and his critics are not just wrong, but their thinking is damaging to Democratic chances of winning the White House and uniting the country in 2020.

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Mark Shields is a nationally syndicated columnist.