Do young people read Joseph Heller’s bitterly brilliant antiwar novel “Catch-22” these days? Given the blizzard of n-words and other anachronistic artifacts clouding its pages, one might guess that it’s out of favor. On the other hand, Amazon (whose founder and chief executive Jeffrey P. Bezos also owns The Post) reports that it remains a favorite dark satire of the madness of killing machines.

There’s a line in it that keeps popping to mind as the 2020 presidential campaign takes shape: “That’s some catch.” In Heller’s book, the hero learns about a key clause in the air corps rule book, called Catch-22. It says that airmen who go mad can ask to be grounded. But given the terror of war, anyone who asks to be grounded is obviously sane. “That’s some catch, that Catch-22,” the hero says. Comes the reply: “It’s the best there is.”

When President Trump rolled out his reelection strategy in his State of the Union message, he declared: “Tonight, we renew our resolve that America will never be a socialist country.” But that, too, comes with a catch: Who defines socialism?

Trump’s determination to have an election about “socialism” crops up in nearly all of his speeches. And the prominence of self-styled socialists in today’s Democratic Party gives him plenty of ammunition. The longtime maverick Sen. Bernie Sanders (I-Vt.) is a front-runner in early polls for the party’s nomination. Nearly half a century younger, Rep. Alexandria Ocasio-Cortez (D-N.Y.) is the party’s newest star. Both call themselves “democratic socialists.”

Because of that catch, that vagueness of meaning, “socialism” may be the best political issue there is — on both sides of the election, stirring hopes among Democrats and fears among Republicans.

For Trump, it papers over a core weakness: Though he’s the head of America’s conservative party, there’s nothing remotely conservative about him. Conservatism respects tradition, views change with caution, and advocates fiscal and personal prudence. That’s not Trump. Opposition to the Utopian impulse of socialism is a conservative position, and Trump can use it to reach beyond his populist base. Independents shaped by the Cold War could be a receptive audience.

For left-leaning Democrats, the same word stirs positive reactions among growing ranks of post-Cold War voters. A recent Harris Poll for Axios found that almost half of voters in the millennial and Generation Z cohorts “prefer to live in a socialist country.” As the influence of younger voters inevitably grows, this sentiment carries more weight at the ballot box.

So that’s some word, “socialism.” It means different things to different people. Before we can have a battle over the idea, there must be a battle over the word itself.

For favorably disposed Democrats, defining socialism starts with saying what it isn’t. It’s not the National Socialism of Germany. It’s not the Soviet Socialism of Stalin. It’s not the Chavistan Socialism currently strangling Venezuela in its death throes. When socialism fails, it ceases to be recognized by socialists. It failed because it wasn’t socialism after all.

On the other hand, socialism is, potentially, any popular government program, from public schools to Medicare. If taxes are collected for a public purpose, and the public approves — voilà! — it’s socialism.

Put these two special qualities of the word together, and the result is wonderfully slippery. Socialists approve a policy smorgasbord from around the world — a Danish program from column A, a Canadian plan from column B and something brewing at a Washington think tank from column C. But they feel no requirement to own any system.

For example, Sanders and Ocasio-Cortez are both fans of Britain’s National Health Service, which provides basic coverage to everyone. But neither dwells on the fact that the British people have resolutely rejected the socialist wing of the Labour Party for the past 40 years, associating it with overpriced energy, violent strikes, stalled trains and rolling blackouts.

Everything good and none of the bad: That’s Democratic socialism. By contrast, Trump and company are counting on a decisive number of voters to make the old connections of government action with bureaucracy, bureaucracy with central planning, central planning with inefficiency, inefficiency with decline and decline with repression.

Historically, socialism has appealed most powerfully in times of upheaval: the Industrial Revolution, mass urbanization, the collapse of empires and so on. People become fearful that change is working against them and seek the promise of large plans and safety nets.

At such times (like now), it’s not enough for defenders of free markets to recite socialist failures. For socialism is always new to its adherents, endlessly reborn in the hope of a more generous future. Trump is more likely to defeat it by expanding opportunity within the free market, rather than by stirring up fears. Unfortunately, fear is what he does best.

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