Socialism, thought to have died years ago, is strangely reviving. Or at least people calling themselves “democratic socialists” are starting to rock the Democratic Party. This first appeared in 2016 with Sen. Bernie Sanders, whose candidacy went surprisingly far. Young people liked the plain-talking, authentic granpa and were intrigued by his democratic socialism.

The latest jolt was the Bronx Democratic primary victory of Alexandria Ocasio-Cortez, a 28-year-old member of Democratic Socialists of America (DSA; Bernie is not). She beat a mainstream incumbent congressman who outspent her tenfold. As usual, turnout was everything, and that requires enthusiasm.

Some establishment Democrats worry that a leftward lurch will spoil the party’s chances for a wave win in November. Yes, it could, but the Democrats better figure out how to incorporate a progressive agenda to retain the millennial vote, the party’s energy for years to come.

Confusion surrounds the term “socialism.” Soviet Communists called their system socialist, meaning state ownership of the means of production under centralized economic control. It declined at an accelerating rate until Gorbachev threw in the towel with a series of stumbling reforms that collapsed the system. The Chinese were far cleverer, but most Chinese production is now in non-state hands, albeit guided by state banks.

Starting in the late 19th century, “revisionist” socialists discarded revolution in favor of elections to gradually introduce welfare measures that uplift the working class. The German Social Democrats (SPD) founded welfarism — a better term than socialism — although it is most developed in Scandinavian “cradle-to-grave” systems. Scandinavians now elect tax-cutting conservatives, and social democratic parties have shrunk across Europe, almost as if the left had been amputated.

Some say welfarism is socialism, but it is really just capitalism with high taxes. One way to estimate the size of a welfare state is its total taxes — around half of GDP in Scandinavia, dropping to some 40 percent farther south in Europe, to under 30 percent in the U.S., the most parsimonious of the industrialized democracies.

Much of U.S. politics is about raising or lowering this percentage. The former is hard, because America was born with the free market — Adam Smith published his “Wealth of Nations” in 1776 — and some still celebrate a perfect-market theory that markets always self-correct.

Then how does one explain the periodic recessions that make everyone gulp in horror? Marx thought they were built into capitalism, which goes through crises of overproduction when workers aren’t paid enough to buy what they produce. Eventually, Marx argued, there will be a depression so big the masses will revolt and install socialism.

Keynes, who sought to rescue capitalism, saw such downturns as simply the normal business cycle that experiences occasional “underconsumption.” He proposed curing it by temporarily boosting aggregate demand through government deficit spending. Debt didn’t bother him (or Donald Trump). Long-term government debt increases, but there’s no clear danger point. Like it or hate it, Keynesianism ain’t socialism.

Many countries carry debt loads that exceed their GDPs. Japan’s is 223 percent of its GDP, the U.S. a modest 78 percent. Neither has damaged their respective economies, although it worries conservatives. Trump’s budget shot up deficits, but his new drastic tariffs work against his tax cut. Tariffs, a tax on imports, flow exclusively to the federal government at the expense of producers and consumers. The damage from tariffs is just starting to hit.

My take on the root of the revival of socialism in America: boredom. The big parties have run out of ideas and élan. People, especially young people, crave something more uplifting than wall-building Republicans and Wall Street Democrats. In cultivating, respectively, evangelicals and the oppressed, both big parties neglect the economic anxiety of most Americans; half lack $400 for an emergency.

By 2020, Trump may be out of MAGA. He offers nothing for economic downturns, trade wars and foreign-policy blunders, especially the Russia connection. Trump’s politics of hatred cultivates his base but alienates independents. Another Hillary-type centrist pastiche may not fill the vacuum. The country needs a vision, which neither Trump nor Hillary offered.

Democratic socialists should prepare a vision before the next recession. If it is as scary as 2008-09 and progressives present a plan to fix it, they may dominate the dialogue. They need coherent policies for stagnant wages, growing inequality, the onslaught of artificial intelligence and environmental degradation.

What democratic socialists lack is another Michael Harrington (1928-89), the brainy and compassionate founder of DSA, who persuaded many non-socialists that poverty still plagued America. This influenced LBJ’s War on Poverty, which some argue was never fully implemented; Vietnam ate the funding for it. A Harrington think tank could renew his challenge.

To reach average citizens may require the revival of a religious left, long eclipsed by the religious right. Increasingly, Christians, galvanized by the mistreatment of immigrant children, distance themselves from Trump and market fundamentalism. Stressful times shift especially younger voters into new party alignments. But who can reach them?