Josh Zeitz has taught American history and politics at Cambridge University and Princeton University and is the author of Lincoln’s Boys: John Hay, John Nicolay, and the War for Lincoln's Image. He is currently writing a book on the making of Lyndon Johnson’s Great Society. Follow him @joshuamzeitz.

One of the great unknowns of the current presidential election cycle—a cycle without recent precedent in its unusual twists and turns—is whether the fall campaign will see a convergence of the populist left and the populist right. Could a chunk of Bernie Sanders’ anti-establishment base really vote for GOP outsider Donald Trump in November? Could it be enough to swing the election in Trump’s favor?

Many experts scoff at the idea. “Not a chance,” former presidential adviser Jim Messina said on Monday. Writing for Politico Magazine, the always perceptive Bill Scher was also skeptical. The “few areas of ideological overlap don’t come close to outweighing the long list of issues where Sanders and Trump are practically opposites,” he wrote.


But here’s the problem with that theory of the case: History shows us that when disenchantment with establishment politics and institutions is high, as it is today, voters don’t always vote along ideological lines.

In 1968 and 1980, insurgent liberal challengers—Eugene McCarthy and Ted Kennedy—captured a popular wave of anti-establishment sentiment but failed to win their party’s nomination. In November, many of their supporters veered sharply to the right, voting for candidates who didn’t necessarily share their political views but who served as a convenient outlet for the expression of their broader frustrations. In both cases, this block of Democratic defectors helped deliver the election to the Republican Party.

It’s a startling historical reminder for Hillary Clinton and her backers as we head into the summer: When voters see America as a nation in decline, and when they attribute the fraying of its economic and social fabric to elite conspiracy, they sometimes cast their votes based on emotion rather than logic.

***

In late 1967 Senator McCarthy, an anti-war liberal from Minnesota, announced he would challenge incumbent President Lyndon Johnson for the Democratic presidential nomination. Out of the gate, his candidacy seemed at best a quixotic adventure. Campaigning in New Hampshire, he skipped scheduled events, refused to make obligatory early-morning appearances at factory gates (“I’m not really a morning person,” he demurred), and delivered dry, uninspiring speeches. When Johnny Carson asked what sort of president he would be, McCarthy replied, “I think I would be adequate.” Even his staff members were unimpressed. Many of them would have much preferred to work for Robert Kennedy, the younger brother of the slain president and an anti-war senator from New York. But Kennedy was not running—yet.

The Tet Offensive, which began on January 31, 1968, changed the political equation almost overnight. As roughly 67,000 Viet Cong troops launched a coordinated assault on South Vietnamese cities, undermining the Johnson administration’s claims that the United States was turning the corner in Southeast Asia, support for the war quickly plummeted. In the unrest that followed, McCarthy stunned political observers by winning 42 percent of the vote in New Hampshire to Johnson’s 49 percent. Days later, Kennedy, sensing Johnson’s weakness, joined the field, and on March 31, LBJ announced his withdrawal from the presidential race and his retirement from politics at the end of his term.

On the surface, McCarthy’s upset in New Hampshire and Kennedy’s late surge in the spring contests—a surge that ended with his assassination in June—suggested an emerging anti-war consensus. But the truth was more complicated. While many of McCarthy’s supporters were genuinely opposed to the war in Vietnam, exit polls showed that a majority thought of themselves as hawks and voted against LBJ to register dissatisfaction with the slow pace of the war effort. Others were unhappy about skyrocketing inflation and urban unrest and simply wanted to register their discontent with the status quo. Johnson’s private pollster found that 55 percent of McCarthy voters supported the conventional bombing campaign against North Vietnam, while only 29 percent opposed it.

The only thing that did seem to unite McCarthy’s supporters was a powerful disdain for the status quo. The country was deeply divided over hot-button issues like racial integration, urban unrest, rising crime rates, inflation, sexual permissiveness, the student movement and Vietnam—all overlapping concerns that generated a fluid political environment.

When the Democratic Party nominated Vice President Hubert Humphrey to run against Richard Nixon in the fall, McCarthy’s supporters didn’t fall neatly in line. Many regarded Humphrey as the establishment’s choice and veered wildly from McCarthy, an anti-establishment liberal, to the only anti-establishment option on the general election ballot—former Alabama Governor George Wallace, an unabashed racist and war hawk who was running as a third-party candidate. Humphrey, whose liberal credentials were unimpeachable, was left uncomfortably grasping to his left and right that fall.

Wallace tapped into a strong reserve of voter anger. Running on a platform of sheer rage—against African-Americans, against crime, against the administration’s failed Vietnam policy, against liberal intellectuals—Wallace whipped traditionally Democratic, working-class audiences into a fury. He dubbed Nixon and Humphrey “Tweedledee and Tweedledum” and claimed there was not “a dime’s worth of difference” between them. He slammed the “overeducated, ivory-tower folks with pointed heads looking down their noses at us” and promised that “when we get to be the president and some anarchist lies down in front of our car, it will be the last car he’ll ever lie down in front of.” Wallace delighted in his exchanges with hecklers and proved deft at playing the generational divide. “You young people seem to know a lot of four-letter words,” he said. “But I have two four-letter words you don’t know: S-O-A-P and W-O-R-K.”

Reports came in from across the Midwest of Wallace rallies packed “wall-to-wall [with] steelworkers” and of union locals whose members were breaking 3-to-1 for the former governor. Democratic labor officials were so concerned that Wallace might siphon off enough votes to deliver key industrial states to Nixon that they began circulating millions of pamphlets to their members, reminding workers that Alabama, where Wallace governed, was a stringently anti-union fortress with antiquated wage and hours laws.

It didn’t hurt labor’s case that Wallace had tapped retired Air Force General Curtis LeMay to be his running mate. LeMay, who famously called for “bomb[ing] the Vietnamese back to the Stone Age,” terrified many Wallace sympathizers into taking a second look at Humphrey. Humphrey, for his part, made good political fodder of the choice, dubbing Wallace and LeMay the “bombsy twins” and emphasizing his own program for peace.

By late fall, many peace Democrats and union members had moved reluctantly back into Humphrey’s column. But ultimately, that didn’t matter: Nixon beat Humphrey by the slimmest of margins. Exit polls showed that roughly 18 percent of McCarthy’s primary voters ended up supporting Wallace. It was enough to swing the election—and it might have been higher, still, had organized labor not intervened.

***

In late 1979, Ted Kennedy—a Massachusetts senator and brother of two assassinated liberal icons—announced he would challenge incumbent President Jimmy Carter for the Democratic nomination.

His campaign got off to a rough start.

In the days immediately following the storming of the American Embassy in Tehran that November, Carter experienced a much-needed bump in the polls. The public closed ranks behind the president, whose approval rating jumped within weeks from 30 percent to 61 percent. In a further boon to Carter, on the same evening the embassy fell, CBS aired a documentary on Kennedy that almost unraveled the challenger’s campaign. After stumbling through tense questioning from newsman Roger Mudd about his conduct at Chappaquiddick, Kennedy badly flubbed what should have been a softball question: Why was he running for president? “Well, I’m—were I to—to make the—announcement,” he began in a halting voice, “is because I have a great belief in this country, that is—has more natural resources than any nation in the world … the greatest technology of any country in the world … the greatest political system in the world. … And I would basically feel that—that it’s imperative for this country to move forward, that it can’t stand still, or otherwise it moves back.”

Kennedy’s performance was shockingly subpar and created a clear impression that he had no particular reason, other than personal ambition, for seeking the highest office in the land. “He was running because he wanted to be president,” Carter’s chief of staff Hamilton Jordan later observed. “That was not such an unusual motive, but most aspirants figure out some way to disguise it better.” As an anxious nation rallied behind its commander in chief, The Boston Globe observed that “the flame under the [Kennedy] charisma has been turned down so low it seems to have gone out, and the motivation seems muffled. … It is a Kennedy campaign without a Kennedy.”

Capitalizing on his newfound jump in the polls, and profiting from his rival’s halting start on the campaign trail, Carter triumphed in the first round of primaries and caucuses, handily defeating Kennedy in the key states of Iowa and New Hampshire. When Carter crushed his opponent in the Illinois primary that spring, most observers expected Kennedy to withdrawal gracefully in favor of the incumbent.

But Carter’s advantage could not last indefinitely. As the president privately admitted to leaders of the House and Senate, patience “is not a characteristic of America.” Each evening, ABC news ran a half-hour update entitled “America Held Hostage,” hosted by veteran newsman Ted Koppel, and newspapers kept Iran on their front pages—all lending the protracted crisis an air of immediate suspense and drama. In late April, a botched military rescue mission left eight servicemen dead and sharply undermined public faith in the president.

After scoring an upset victory over Carter in delegate-rich New York, Kennedy recovered his poise and won consecutive contests in Pennsylvania, Arizona, New Mexico, California, New Jersey, Rhode Island, South Dakota and Michigan. Exit polls revealed that his primary supporters were disillusioned by the president’s inability to resolve the hostage crisis but equally preoccupied by economic concerns including fuel shortages, a declining manufacturing sector and runaway inflation. As a journalist observed, the electoral coalition that Kennedy managed to assemble late in the primary season was precisely the alignment his brother Robert had attempted with great fanfare but less success to cobble together in 1968: “blacks and liberals and blue-collar conservatives, all united by their anger at the way things were.”

It was too late. By the time Kennedy recovered his early momentum, the president had already won enough delegates to secure the nomination on the first ballot. He had emerged from the contest badly wounded, but victorious.

On Election Day, voters delivered a stunning victory to Ronald Reagan. He won 50.7 percent of the popular vote to Carter’s 41 percent and independent Congressman John Anderson’s 6.6 percent. In the winner-takes-all Electoral College, Reagan ran away with 489 votes to Carter’s 49.

Given the scope of their victory, Republicans had reason to interpret the 1980 elections as a moment of electoral realignment. But the truth was more complicated. Voters had repudiated Carter, but they didn’t embrace Reagan. Veering wildly from left to right, as they had in 1968, ordinary citizens longed desperately for strong, decisive leadership. The challenges of stagflation and energy shortages, the dislocations of Watergate, Vietnam and the civil rights revolution produced a powerful range of backlash and sentiment that could not be defined strictly as “liberal” or “conservative.” On Election Day, some 27 percent of Ted Kennedy’s primary supporters cast their votes for Reagan. Exit polls showed that supporters of Anderson, who was arguably the most liberal candidate in the general election field, would have divided evenly between Carter and Reagan had their man exited the race. The results were effectively less an expression of conservative ascendance than popular distress.

“The question is not which one is best,” a voter in Akron, Ohio, observed in 1980, “but which one is the least worst.” A minister in Massachusetts agreed, taking solace in the knowledge that “only one of them can get elected!” A New York Times/CBS exit poll revealed that 38 percent of Reagan voters cast their lot with the former California governor because they believed it was “time for a change.” Only 11 percent voted for him because “he’s a real conservative.” “It’s the first time I ever voted Republican,” said a Michigan resident. “But I’m sick and tired of the mess that’s going on in this country.”

Even after the results were in, a Time survey indicated that almost two-thirds of Americans interpreted the election as “mostly a rejection of President Carter and his administration” rather than “a mandate for more conservative politics.” Most seemed to agree with a Virginia voter who thought both candidates were “at the bottom of the list. … Carter is inept at making decisions. … Reagan is inept at thinking, period.” Still, after a long and challenging decade—one that saw a restless and diverse nation come to grips with the aftershocks of the 1960s—many people hoped that the tanned and genial actor from Hollywood could restore America to its former greatness. Certainly they were ready to give him a try.

“I’m a liberal,” boasted a white ethnic voter from Queens shortly before the election. “Carter’s a disaster. I think what the country really needs is a father figure, so I’m voting for Ronald Reagan.”



***

For all their divergent beliefs, Donald Trump and Bernie Sanders have each tapped into raw anger and resentment that is in some ways more emotive than ideological. The dangers for Hillary Clinton are clear. By most reasonable standards, she is as unimpeachably liberal as Humphrey was in 1968, yet she is equally distrusted by the anti-establishment left. She will need to guard against defections to an anti-establishment conservative who has proved every bit as deft as Wallace.

And, like Carter in 1980, Clinton will enter the fall campaign with sky-high disapproval ratings, in no small part because her primary opponent spent a year casting her as an enemy of the common man. True, Trump is also wildly unpopular. But people tend to forget that Reagan was hardly more trusted when he unseated Carter than Trump is today—and that year, voters chose the candidate who represented a break with the status quo.

Citing survey data from earlier this year, political scientists Christopher Achen and Larry Bartels recently observed that “supporters of Mr. Sanders were more pessimistic than Mrs. Clinton’s supporters … and more likely to say that economic inequality had increased. However, they were less likely than Mrs. Clinton’s supporters to favor concrete policies that Mr. Sanders has offered as remedies for these ills, including a higher minimum wage, increasing government spending on health care and an expansion of government services financed by higher taxes.” Achen and Bartels attribute Sanders’ appeal to identity politics, particularly given his disproportionate traction with “disaffected white men.” In this respect, the Sanders electorate is not dissimilar from a large portion of McCarthy’s and Kennedy’s supporters.

Of course, America is a different place in 2016. It is less white, less native-born, and more culturally permissive than in 1968 or 1980. A candidate who proposes to break up millions of families through mass arrests and deportations, and whose campaign has earned the adulation of leading white supremacists, begins with an electoral disadvantage. A candidate who wears his misogyny like a badge of honor is poorly positioned to capture the millions of women who have gathered under Sanders’ banner.

But what if 20 percent of Sanders voters in key states defect to Trump, as they did in previous years? Could it be enough to swing an election?

The historical lesson for Hillary Clinton is clear: Watch the left flank, because it could very well swing to the right.