"Those who cannot remember the past are condemned to repeat it." Attributed to writer and philosopher George Santayana.
1964 election campaigns were a mirror-image to what we are experiencing now in 2020. As with any mirror-image everything that was left became right, and everything that is right became left.
In 1964 the incumbent president was a Democrat whereas we have a Republican incumbent now. The eventual Republican challenger was Barry Goldwater. If Bernie Sanders were to become the Democratic Party's nominee, we have historical parallel which tells us how the contest will play out next November.
Goldwater was the reverse of Bernie Sanders politically, but both candidates have much in common. Both men openly disdained the moderates in the political parties of which they sought the presidential nomination.
At the GOP convention in San Francisco's Cow Palace, Goldwater uttered his most famous phrase: "I would remind you that extremism in the defense of liberty is no vice. And let me remind you also that moderation in the pursuit of justice is no virtue."
Likewise, the famously uncompromising Bernie Sanders refuses "to accept incremental, half-a-loaf-is-better-than-none politics and demanding go-for-broke maximalism instead", as Paul Krugman of the New York Times observed.
Other than the fact that the Goldwater and Sanders were political polar opposites, both men have been described in similar terms. Fellow Arizonan, John McCain, wrote in his autobiography that Barry Goldwater was "an authentic maverick", "irascible and principled, fiercely independent and deeply patriotic." The Los Angeles Times wrote, "Barry Goldwater, the blunt-spoken, charismatic senator and failed presidential nominee who hurled conservative rhetoric... He was 89." The New York Times reported that at his funeral in 1998, Goldwater was "hailed as a man of honesty and principle to a fault".
If you replace "conservative" with "progressive", the adjectives above describing Barry Goldwater can easily be applied to Bernie Sanders.
The Republican Party was divided in 1964 between its conservative and moderate-liberal factions. Goldwater was the champion of the conservatives. The conservatives favored a low-tax, small federal government which supported individual rights and business interests and opposed social welfare programs. The conservatives also resented the dominance of the GOP's moderate, establishment wing, which was based in the Northeastern United States. The conservatives believed the Eastern moderates were little different from liberal Democrats in their philosophy and approach to government. Goldwater's chief opponent for the Republican nomination was Nelson Rockefeller, the Governor of New York and the longtime leader of the GOP's liberal-moderate faction.
To many GOP moderates, Goldwater's convention speech, especially his "...is no vice, ...is no virtue" parallel antithesis, was seen as a deliberate insult, and many of these moderates would defect to the Democrats in the fall election.
Although Goldwater had been successful in rallying conservatives, he was unable to broaden his base of support for the general election. Lyndon Johnson positioned himself as a moderate and succeeded in portraying Goldwater as a right-wing extremist. In much the same way Donald Trump portrays Sanders as a left-wing extremist.
Goldwater had a habit of making blunt statements about war, nuclear weapons, and economics that could be turned against him. Sanders similarly has habitually made statements that seem to praise Communist leaders and Sanders unapologetically has praised socialist economic policies. If Sanders were to become the Democratic nominee, Trump and the Republicans will certainly dredge up anything Sanders has ever said that is even vaguely complimentary of communists and communism.
The election was held on November 3, 1964. Johnson beat Goldwater in the general election, winning over 61% of the popular vote, the highest percentage since the popular vote first became widespread in 1824. In the end, Goldwater won only his native state of Arizona and five Deep South states—Louisiana, Mississippi, Georgia, Alabama and South Carolina—which had been increasingly alienated by Democratic civil rights policies.
Bernie Sanders has a good shot at carrying Vermont if he were a general election candidate. His winning any states beyond that is highly speculative.