To be a ‘‘liberal,’’ in this account, is in some sense to be a fake. It’s to shroud an ambiguous, even reactionary agenda under a superficial commitment to social justice and moderate, incremental change. American liberalism was once associated with something far more robust, with immoderate presidents and spectacular waves of legislation like Franklin Roosevelt’s New Deal and Lyndon Johnson’s Great Society. Today’s liberals stand accused of forsaking the clarity and ambition of even that flawed legacy. To call someone a liberal now, in other words, is often to denounce him or her as having abandoned liberalism.

Liberal-bashing on social media has reached a kind of apogee, but its targets have not yet produced much real defense of the ideology. This means the word ‘‘liberal’’ is, for the moment, almost entirely one of abuse. It is hard to think of an American politician who has embraced it, even going back two or three generations. If liberalism is dead, then, it’s a strange sort of demise: Here is an ideology that has many accused sympathizers, but no champions, no defenders.

America’s version of liberalism has always been a curious one. In Europe, the word has traditionally meant a preference for things like limited government, separate private and public spheres, freedom of the press and association, free trade and open markets — what’s often described as ‘‘classical liberalism.’’ But the United States had many of those inclinations from the beginning. By the 20th century, American liberalism had come to mean something distinct. The focus on individual liberties was still there, but the vision of government had become stronger, more interventionist — ready to regulate markets, bust monopolies and spend its way out of economic downturns. After the end of World War II, this version of liberalism seemed so triumphant in the United States that the critic Lionel Trilling called it the country’s ‘‘sole intellectual tradition.’’ Its legislation legalized unions and, with Social Security, created a pension system; a health plan for older Americans, Medicare, was on the way.

But as these same liberals initiated anti-Communist interventions in Korea and Vietnam, or counseled patience and moderation to civil rights activists, they quickly found themselves in the same position we see today: under heavy abuse from the left. In a landmark speech at an antiwar rally in April 1965, Paul Potter, the president of Students for a Democratic Society, asked: ‘‘What kind of system is it that justifies the United States or any country seizing the destinies of the Vietnamese people and using them callously for its own purpose? What kind of system is it that disenfranchises people in the South?’’ The first step, as he saw it, was clear: ‘‘We must name that system.’’ In a speech later that year, his successor as S.D.S. president, Carl Oglesby, did precisely that, calling it ‘‘corporate liberalism’’ — an unholy alliance of business and the state that was enriching to elites but destructive to working-class Americans and the world’s poor.

It was the 1980 victory of Ronald Reagan and his brand of conservatism that set in motion the villainizing of American liberalism from the right — this time not for warmongering but for supposedly being soft on crime and communism, bloating the government with ineffective social programs and turning American universities into hothouses of fetid radicalism. Many demoralized liberals responded by abandoning the label completely. The nasty 1988 presidential campaign may have been a watershed. In one debate, Bush demanded that his opponent, Gov. Michael Dukakis of Massachusetts, explain ‘‘some of these very liberal positions.’’ Dukakis’s reply, a weak ‘‘Let’s stop labeling each other,’’ only confirmed the word as an insult. A few weeks before the election, dozens of distinguished figures — from novelists to editors to former Secretary of Defense Robert McNamara — bought a full-page ad in The Times to print a letter titled ‘‘A Reaffirmation of Principles,’’ expressing their alarm at the use of ‘‘liberal’’ as a term of ‘‘opprobrium.’’ But their own definition of it was oddly vague: They called it ‘‘the institutional defense of decency.’’ All those attacks on liberalism seemed to be weakening people’s sense of what liberalism even meant.