Sen. Bernie Sanders (I-Vt.) speaks in support of Brent Welder during a rally at the Reardon Convention Center on July 20 in Kansas City, Kan. Welder went on to lose a primary election. (Dan Videtich/For The Washington Post)
David Greenberg is a history professor at Rutgers University. His books include “Republic of Spin: An Inside History of the American Presidency” and “Nixon’s Shadow: The History of an Image.”

“I am not a member of any organized political party,” the early 20th-century comedian Will Rogers once said. “I am a Democrat.” Nearly a century later this quip remains apt. This fall, when the party looks likely to retake control of the House of Representatives and has an outside shot to win the Senate, Democrats seem to be delighting in fighting one another.

To hear the pundits tell it, the cleavage runs deep between the party’s “progressive” and “moderate” factions. But this understanding, and the terminology that goes with it, is wrong. In truth, few moderate Democrats remain in national office. The split is now a different one: between liberals and leftists. Understanding this distinction matters as voters assess the candidates vying to lead the party during — and after — the Trump presidency.

This confusion occurs because for two generations, party fault lines did in fact run between liberals and centrists.

Starting in the turmoil of the late 1960s, Democrats — once the nation’s majority party — found themselves on the defensive. From 1968 to 1988, they lost every presidential race but one (Jimmy Carter’s razor-thin post-Watergate win in 1976), several of them in landslides, and lost ground in state and local contests as well. The party’s support for civil rights had caused the once-Democratic South to gravitate slowly but inexorably toward Republicans, while its leftward shift on foreign policy, crime, welfare and other cultural issues alienated working-class whites elsewhere — Nixon’s “Silent Majority” and the vaunted “Reagan Democrats.”

Many concluded a more pragmatic politics was needed to regain power. In 1972, after the party nominated the ultraleft Sen. George McGovern of South Dakota, a group called the Coalition for a Democratic Majority tried to unite Democratic moderates but made little headway. Then in 1985, the Democratic Leadership Council emerged as a far more effective successor groups.

The Democratic primaries of 1984 exposed the split, with the old-line liberal Walter Mondale facing off against youthful Sen. Gary Hart of Colorado and his self-proclaimed “new ideas.” (“Where’s the beef?” Mondale famously asked.) The 1988 field contained both unreconstructed New Dealers like Paul Simon of Illinois and more moderate DLC figures like Bruce Babbitt of Arizona and Al Gore of Tennessee.

Tellingly, the winner was Massachusetts Gov. Michael Dukakis, a technocratic liberal who straddled both camps. But the Republicans slimed him as an old-fashioned liberal — soft on crime, weak on foreign policy and insufficiently patriotic. Only in 1992 did Bill Clinton, whose Southern charm immunized him somewhat from being saddled with damaging liberal stereotypes, succeed in forging a pragmatic synthesis that could unite liberals and DLC types.

Notably, the leftmost aspirant that year, Sen. Tom Harkin of Iowa, got zero traction, allowing Clinton to run to the left of his main foe, the pro-business centrist Paul Tsongas. Thus defining himself against right and left, Clinton won back the Reagan Democrats and many upscale Republicans. He left office eight years later as the most popular outgoing president since polling began.

The Clinton blend of liberal and centrist approaches held for a quarter century. Party nominees Al Gore, John F. Kerry, Barack Obama and Hillary Clinton all more or less followed his model of positioning the party as the champion of working families, yoking growth economics to a strengthened social safety net. (Hillary Clinton failed to stress her economic program, but it was in this liberal vein.)

Meanwhile, the old McGovern wing of the party atrophied, leaving few 1960s-style leftists on the national scene. The Clinton administration’s many successes — achieving broadly shared prosperity and deficit reduction, making huge strides against crime, teen pregnancy and other social problems, improving race relations and reestablishing America as a force for good in the world — deprived the left of a compelling, urgent agenda. The meager support for Ralph Nader’s 2000 spoiler bid showed how irrelevant the far left had become.

But the far left wasn’t the only faction that crumbled during this period.

So, too, ironically, did the DLC. True centrists like Sen. Sam Nunn of Georgia and Sen. David Boren of Oklahoma retired from politics, and others joined the Republican Party. Other figures like Gore and former congressman Richard Gephardt of Missouri, once associated with the “moderate” wing of the party, moved leftward on certain issues to win favor from the liberal base. By the Obama years, Blue Dogs, a caucus of U.S. representatives who identify as conservative Democrats, remained mainly in those states and districts where no liberal could possibly compete, and wave elections in 2010 and 2014 wiped out many of those that remained. In a sign of the times, the DLC disbanded in 2011.

In the new century, two events gradually led to disillusionment with the Clinton-Obama formula. First, many sitting Democratic senators in 2003 supported the invasion of Iraq, upsetting the party’s base. Five years later, the crash of 2008 and ensuing recession triggered a demand for more populist economics, notwithstanding Clinton’s and Obama’s emphases on a fairer tax structure, expanded health care and other redistributionist measures. Many on the left seethed that financial firms got bailed out, but Obama and his Wall Street-friendly treasury secretary, Tim Geithner, declined to undertake a large-scale bailout of homeowners or take a more punitive approach toward the banks.

In 2016, the backlash began in earnest. The forces fueling the surge of Sen. Bernie Sanders (I-Vt.) in the presidential primaries that year remain poorly understood; considerable evidence suggests his voters weren’t on the whole more left wing than Hillary Clinton’s, but disliked her for other reasons.

Still, especially among the young, there was obviously a hunger for stronger policies to check business and finance, larger federal benefits and a bolder rhetorical attack on established institutions. The Sanders candidacy brought new, younger, left-leaning voters into the political process and nurtured in them a hostility to the Democratic Party as an institution. (Sanders himself, despite joining the party to compete in the primaries, left it again after the race  — sending a powerful signal about the low esteem in which he believed it should be held.)

In waging his campaign, Sanders and his supporters tarred Clinton as lacking “progressive” bona fides — as that perpetually malleable term morphed from a euphemism for liberal to a euphemism for a more radical leftist.

Since then, the Sanders movement has tried to move the party leftward, moving the goal post of acceptably left opinion. When House Minority Leader Nancy Pelosi (D-Calif.) entered the Democratic leadership in the early 2000s, she was understood as representing the party’s more progressive wing. Yet today she faces a rebellion from her left flank. Tom Perez, too, was cheered as a liberal hero for his work in the Obama Justice and Labor departments — until he faced off against the further-left Keith Ellison for the Democratic National Committee chairmanship and drew the scorn of the radicals.

This year, that playbook is being employed in primary after primary. Mainstream liberals find themselves caricatured as establishment centrists who lack commitment to the Democratic Party’s time-honored values. And yet these Democrats are actually just dyed-in-the-wool liberals in the tradition of Franklin Roosevelt.

Liberals, not moderates. That’s because, as the short-lived 2016 presidential candidacy of Jim Webb showed, moderates barely survive in the party anymore — with the important exception of those in red states and districts like West Virginia’s Sen. Joe Manchin III or North Dakota’s Sen. Heidi Heitkamp. And even they confront grumbling and pressure from the far-left elements of the Democratic base.

The result is that the upstarts are routinely targeting reliable liberals who have served their constituents well. Queens panjandrum Rep. Joseph Crowley, whose defeat this summer was widely cheered, boasted stellar ratings from almost every liberal policy group, from Planned Parenthood to the League of Conservation Voters to the ACLU. Equally solid in their liberal credentials were Democrats who prevailed over left-wing challenges in ugly primaries, such as gubernatorial candidates Richard Cordray in Ohio and Gretchen Whitmer in Michigan.

Several more races like this remain. Those determined to villainize long-serving politicians could easily cherry-pick the records of California’s Sen. Dianne Feinstein, New York’s Gov. Andrew M. Cuomo and Massachusetts’s Rep. Michael E. Capuano — all of whom the left has in its gunsights — to find wayward votes or policies. But no honest description would classify these liberals as Manchin-like centrists.

Some policy differences divide the leftists from the liberals, such as whether to abolish or merely reform the immigration enforcement agency ICE, and debating those issues is legitimate and in some cases important. But the real divide between liberals and leftists is attitudinal.

The former tend to support the men and women who have been leading the Democratic Party, believing their goals have been thwarted mainly by Republicans. They understand that incrementalism and compromise are sometimes necessary. The latter tend to blame the Democrats themselves, believing that greater zeal might somehow have vanquished conservative opposition to the liberal agenda — or that Democrats simply didn’t set their sights high enough.

It is probably inevitable that disorganized Democrats will wage an internal war, in 2018 and beyond, over these basic questions of whether it is to be a liberal or a radical party. But the war should be fought on those terms — and not as though it’s craven, corporate centrists who are holding the levers of power.