Yet even Sanders' revolutionary quest could not break through one remaining barrier for the party: the use of the term "liberal."
The Vermont senator and and former secretary of state duked it out on the trail over many positions that put them squarely in the liberal camp. But the word they used in their battles was "progressive," not the loaded L-word.
Sanders, a self-proclaimed democratic socialist, frequently set the terms of debate on the left end of the spectrum. He argued that a candidate truly dedicated to progressive causes wouldn't have taken big donations from Wall Street, flip-flopped on the Keystone XL pipeline or voted for the Iraq War as Clinton did.
After only narrowly winning the Iowa caucuses on Feb. 1, Clinton fought back as a "progressive who gets things done for people." She portrayed herself as equally committed to causes dear to progressives' hearts but more realistic about how to tackle political obstacles, like a Republican-controlled Congress.
That parsing reflects an ongoing, if belated, effort by Democrats to shift the political language back in their favor. Democrats have political scar tissue from being repeatedly pummeled as liberals -- including three straight crushing presidential defeats in the 1980s. It's a label so toxic after decades of Republican attacks that even the most ardent Democrats run away from it.
Compare that approach to the Republican presidential field, where every day White House hopefuls tried to out-"conservative" each other. At the February 13 GOP debate, then-candidate Jeb Bush called himself a conservative
six times, with Donald Trump and Ted Cruz just behind and the other Republicans invoking it frequently as well.
Since the mid-1960s, conservative politicians, activists and media outlets have shaded "liberal" with an ominous and sinister connotation. Upheaval in the 1960s spurred this on, as conservatives portrayed Democrats as libertine hedonists, permissive to a fault and at odds with Middle America's clean-cut values.
Fair or not, Democrats generally did little to sever the term with that image.
"Democrats fled the word 'liberal' as it got demonized," said Bob Shrum, for decades an adviser on Democratic presidential campaigns. "They didn't fight back; they gave it up."
They did, however, more recently engage in some linguistic counter-programming, according to Rep. Steve Israel of New York, who chairs the House Democrats' messaging arm.
"In 2010, 'tea party' was a very shiny and popular brand. What we've done is brought the name tea party into negative territory," he told CNN. "Now a lot of Republicans lead with disclaiming and distancing themselves from the tea party."
Shrum, currently a professor of practical politics at the University of Southern California, also said the term "conservative" could potentially be a rhetorical target for Democrats, especially with Trump, the presumptive Republican presidential nominee, intermittently describing himself that way.
"I think conservatism in this election cycle has actually morphed from a set of principles and programs to a set of grievances and anger. Trump expresses that," he said.
Democratic consultant Doug Hattaway said there was never a conscious effort by members of his party to not defend the "liberal" term. Moving away from it was the path of least resistance.
"Rather than defining the word in a positive way, Democrats allowed Republicans to tarnish it," Hattaway said. "Democrats need to either re-define the term or adopt 'progressive' in a consistent way. 'Progressive' is seen as a positive word, but it's not clearly defined, so there might be an opportunity for Democrats to own it."
Being called a liberal wasn't always so politically toxic for Democrats. In the immediate post-World War II years, with New Deal memories still fresh for millions of Americans who lived through the Great Depression, the term was used proudly. Liberals saw themselves as accepting the idea of progressive change and being forward-looking in a rapidly evolving world.
In President Harry Truman's 1948 campaign, he frequently called himself a liberal, noted Sharon E. Jarvis, a University of Texas professor and author of "The Talk of the Party: Political Labels, Symbolic Capital, and American Life." Truman told one audience that he was "happy to pay tribute to the liberal spirit of the forces of labor in the United States" and wanted "to reiterate emphatically [his] faith in the liberal philosophy of the Democratic Party."
In his 1959 inaugural address, California Gov. Pat Brown used the word "liberal" or "liberalism" seven times in the first eight paragraphs. The catchphrase of his Democratic administration was "responsible liberalism."
President John F. Kennedy similarly defined a liberal as someone who "cares about the welfare of the people -- their health, their housing, their schools, their jobs, their civil rights and their civil liberties."
By that definition, he added, "I'm proud to say I'm a liberal."
More than a half-century on, though, Democrats demur in describing themselves that way.
Israel, who is retiring at the end of this term after 16 years in Congress, said making "liberal" a political bad word was part of an upper hand Republicans gained in communications strategy.
"Republicans developed this message brand of 'tax-and-spend (liberals),' and got very disciplined on it," he said. "You could be talking about the Mets vs. the Dodgers, and good Republican operatives would be able to weave in tax-and-spend."
From Reagan to Gingrich
Throughout President Ronald Reagan's two terms, conservatives pummeled Democrats with the "liberal" label, using it as shorthand for being soft on communism and achingly sensitive to minority grievances.
Reagan himself, usually known for his upbeat rhetoric, jabbed the opposition with the liberal tag at the 1988 Republican National Convention.
The president decried
"liberal, liberal, liberal" policies of the Democrats. "Every single year I've been in office, I have supported and called for a balanced budget amendment to the Constitution, and the liberals have said no every year," Reagan told the uproarious crowd in a typical refrain. "I called for the line-item veto, which 43 governors have, to cut fat in the budget, and the liberals have said no."
"Reagan sneered the word 'liberal' or 'liberalism' 22 times in a 25-minute address," The Los Angeles Times reported
And it seemed to work, as his vice president, George H.W. Bush, won the White House echoing much of this rhetoric.
No other Republican figure, however, probably did more to make Democrats run scared from "liberalism" than a one-time backbench Republican congressman from Georgia, Newt Gingrich.
As House minority whip in 1990, his political action committee GOPAC sent out a memo
titled "Language: A Key Mechanism of Control" to several thousand Republican candidates. It advised them to use a series of pejoratives against Democrats. "Liberal" was among the most prominent, along with "decay," "destructive," "pathetic" and "corrupt."
A 'liberal' comeback?
After Republicans in 1994 won control of the House for the first time in 40 years, Gingrich, now speaker, had a much more prominent perch from which to tag Democrats as dangerous liberals. So did fellow Republicans who took over the Senate.
A typical ad ran in 1996 against Democratic Sen. Paul Wellstone of Minnesota. Wellstone's Republican opponent declared on the TV screen that Wellstone was beyond liberal, an "ultraliberal" and, finally, "embarrassingly liberal."
The attacks didn't work, however, as Wellstone won re-election. And the Gingrich era may have marked the high point for the effectiveness of slams on Democrats as liberals.
President Bill Clinton had something to do with it, basing his 1996 re-election on a "triangulation" strategy and positioning himself between ideological extremes of the left and right. That included signing a welfare-overhaul bill pushed by congressional Republicans while rejecting GOP demands to cut Social Security and Medicare benefits.
The term "liberal" still pops up in Republican campaign rhetoric, though far less frequently than it used to. Overall the phrase seems less potent.
According to a recent Pew Research Center poll
, Democratic voters are increasingly identifying with the label liberal. "In 2015, more Democratic voters identified as liberals (42%) than as moderates (38%) or conservatives (17%)," reported Pew.
According to Jeremy Mayer, associate professor in the School of Policy, Government and International Affairs at George Mason University, "American politics is so polarized now that anyone who is scared of 'liberal' is already voting for the Republican nominee. To rally the base of your party, use the most vivid, gripping language that doesn't set off independents too much."
With five months to go before voters decide whether Hillary Clinton or Donald Trump will be the 45th president, Democrats may for the first time see political advantage in calling themselves "liberal." After all, a rapidly diversifying demographic voting base -- which twice put Barack Obama in the White House -- may identify with the phrase.
But based on recent political history, don't expect to hear it in Hillary Clinton's acceptance speech at the Democratic convention in Philadelphia. Or anywhere else.