In every presidential race, there invariably comes a moment when the pundits and predictors realize, to their horror, the favorite could lose. That moment came in the past week for Hillary Clinton.

It wasn’t a single event, but rather the slow chipping away of her aura of inevitability until she was revealed as a political mortal like everyone else. One factor was her uneven debate performance Sunday night that was only saved by her passionate invocation of the Flint, Mich., water crisis. Also contributing to the media buzz kill was a new CNN/WMUR poll that showed her losing New Hampshire by almost 2 to 1.

These are, quite possibly, just blips. All New Hampshire  polls should come with the reminder that 48 percent of Granite State Democrats said in 2008 exit polls they had only made up their minds in the past week. To update Mark Twain: “If you don’t like the polling in New Hampshire, just wait a few minutes.”

Since the Republicans anointed Vice President Richard M. Nixon in 1960, no one (incumbents aside) has been handed a presidential nomination without a fight.

Still, it defies political logic (that is, 2014 Conventional Wisdom) that Clinton is struggling to fend off a socialist back-bench senator who is funding his campaign mostly through small contributions. And, oh yes, that senator is Jewish and the oldest serious candidate (he’ll be 75 on Inauguration Day) ever to seek the presidency.

For all its surface oddity, the battle between Clinton and Bernard Sanders reflects divisions in the Democratic Party that go back more than half a century.

It is a choice between a pragmatic candidate (Hillary) and an idealistic one (Bernie).

This Democratic split dates back, at least, to the 1960 Democratic Convention that rejected the dreamy wistfulness of Adlai Stevenson for the hard-edged realism of John F. Kennedy. For antiwar Democrats in the Vietnam-protest campaign of 1968, the choice was between a courageous outsider (Eugene McCarthy) and the supposedly ruthless Robert Kennedy. In many ways, the mercurial McCarthy was the more conservative of the two candidates, but symbolically he represented the air-castle wing of the Democratic Party rather than its Machiavellian side.

There are other parallels: George McGovern in 1972 defeated the Democratic Party machine and then went down to a 49-state defeat. (“Don’t Blame Me, I’m From Massachusetts” was a popular bumper sticker during Watergate). Echoes of these same idealist-pragmatist tensions can be found in Gary Hart’s 1984 challenge to Walter Mondale and Bill Bradley’s quixotic 2000 crusade against incumbent VP Al Gore. And, of course, Barack Obama versus Clinton in 2008 fit this pattern.

These were, at their core, stylistic struggles rather than the ideological fights that have roiled the GOP. Ever since the Democratic Party shed the segregationist South and lost socially conservative blue-collar voters during the Reagan years, the party has been far more united on issues than the Republicans. Instead, what continues to divide the Democrats is temperament and the willingness to temper dreams to suit a poll-driven view of what is politically possible.

Harking back to the Kennedy-McCarthy 1968 struggle for the hearts and minds of the antiwar movement may seem like ancient history. But both Sanders and the 68-year-old Clinton have long memories.

What many younger reporters (who heard about the 1960s from their parents and barely remember the Clinton years) fail to grasp is how long-ago events molded the world views of the two leading Democrats.

To understand Sanders, it is necessary to grasp how much Marxism was embedded in the left-wing debates of the early 1960s, especially at schools like his alma mater, the University of Chicago. This was not — please, please understand — an endorsement of Soviet communism, which was seen as Stalinist and sclerotic. But the economic determinism of Marx and Engels produced a belief that Wall Street manipulated events. 

And youthful yearning led to the romanticizing of lost-cause American socialists such as Eugene Debs, who ran for president five times and in 1920 received nearly 1 million votes.

In moving to Vermont in the late 1960s, the Brooklyn-bred Bernie was beguiled by back-to-the-land utopianism. Living in an unheated sugar house was also a way to drop out from the television-saturated middle-class consumer culture that defined America at the mid-century.

Hillary Rodham — an antiwar commencement speaker at Wellesley — was shaped by the feminism that emerged at elite schools in the late 1960s. Talking to long-time friends for an Esquire profile in 1993, I realized her life would probably have been different if she had been born a bit earlier (dabbling in citizen politics as the wife of a partner in a Chicago law firm) or a bit later (refusing to move to Arkansas to advance Bill Clinton’s political career rather than her own).

You can still occasionally hear whiffs of that original Hillary on the campaign trail. I recall a stump speech at Haverford College before the 2008 Pennsylvania primary when — out of nowhere — she started riffing about her horror over the women’s dress-for-success look of the 1980s. That ill-conceived fashion trend was defined by copying Brooks Brothers menswear down to a floppy bow at the neck. Hillary made clear that a floppy bow was the last thing that she would ever wear to the Rose Law Firm in Little Rock.

Hillary Clinton, the politician, has been defined by something else as well — deeply ingrained caution. I always thought that her mistrust of the press dated back to the 1992 campaign, when she came under fire for potential conflicts of interest in her legal work in Little Rock while her husband was governor. She compounded the problem by snapping, “I could have stayed home and baked cookies and had teas, but what I decided to do was fulfill my profession.”

In the context of the 1992 campaign, this “baked cookies” remark was seen as an affront to stay-at-home mothers. And Clinton — so used to her marital role as the one who cleaned Bill’s messes — suddenly created a firestorm on her own.

Her political caution also dates back to the three devastating presidential defeats the Democrats suffered in the 1980s. Only 11 states went Democratic in any of those three elections. As a result, Clinton is more sensitive than almost any Democrat (exception: Charles E. Schumer) to how the Republicans will respond to any policy proposal.

Bill Clinton’s presidency was, in effect, the belated Democratic response to Ronald Reagan by taking three crippling issues off the table: welfare, crime and the deficit. The failure of Hillary Clinton’s think-big 1994 health care plan contributed to her current belief in the power of pragmatism and the idealism of incrementalism.

It is easy to imagine Clinton’s exasperation with Bernie Sanders’ vaporous belief that his election would make Wall Street crumble. At the same time, you can picture the frustration of Sanders at the timidity of the Democrats — a party he only deigned to join for this campaign.

My instinct is that the Bernie brouhaha will fade once Democratic voters realize they’re electing a president — and not sending a message.

But this is a strange political year. And it may have been defined by Bob Dylan — that rare musical figure that both Clinton and Sanders probably know better than their pop-culture-obsessed aides.

In 1965 Dylan wrote, “Because something is happening here/But you don’t know what it is/Do you, Mister Jones?”

That bafflement is equally potent 50 years later.