Defining the word “progressive” is a lot like defining pornography—you just know it when you see it. Or so Walter Mondale suggested while stumping for Jimmy Carter at a rally in Syracuse in 1980. At a time when the president was taking heat from liberal critics, Mondale reassured the crowd that he “knows a progressive when he sees one.”

In today’s Democratic primary field, everybody claims to see a progressive looking back at them in the mirror. Bernie Sanders has perhaps the soundest claim to the title; he co-founded the Congressional Progressive Caucus in 1991. But Kamala Harris also brands herself a “progressive prosecutor”; Cory Booker’s “progressive record is unimpeachable,” says his spokesperson; and even Joe Biden claims “the most progressive record of anybody.”

The most consistent thing about the political application of the word may be its inconsistency. The progressive tent has sheltered Theodore Roosevelt, who championed a “progressive” income tax while bringing “progress” to the Philippines, along with the water cure and concentration camps. The socialist Eugene Debs, who went to prison for protesting World War I, was called a progressive, as was the bigoted Woodrow Wilson, who took the United States into World War I. Later, liberals and Communists both laid claim to the term, so “progressive” might have described both Frank Sinatra (a supporter of Henry Wallace, a New Dealer who ran for president as a Progressive in the 1948 election) and Joseph Stalin (the “Leader of Progressive Mankind,” said Pravda).

One reason for the word’s inconsistent use lies in its root word, “progress.” Like “reform,” progress is broadly appealing without being politically exacting. To believe in “progress” commits you only to a vague belief in improvement over time. The early progressives assumed they could achieve these “improvements” through data, science, and institutional expertise; their veneration of administrative efficiency led them to support conservation, which was one progressive value, as well as eugenics, which was another. Many believed deeply in American empire—colonization, they thought, wrought progress.

Despite its slippery evasiveness, or perhaps because of it, the term took on a new purpose in the 1980s. When Michael Dukakis called himself a “liberal” late in his failed 1988 campaign, George Bush impishly cheered, “My opponent finally, after knocking me in the debate, called himself the big ‘L.’” “Progressive” became an innocuous synonym for the increasingly taboo L-word. The Oxford English Dictionary is uncharacteristically blunt about this political reality: “In the United States now often used as a self-designation by people on the left to avoid the term liberal.” Still mindful of the dangers of “liberalism,” Pete Buttigieg says, “I view myself as a progressive, but these labels are becoming less and less useful.”