Here’s a question tormenting Democratic officials and liberal pundits as the country hurtles toward Election Day: When did young voters stop liking them?

It was bad enough for them that millennials backed Bernie Sanders in unprecedented numbers over party favorite Hillary Clinton during the presidential primary. But now the kids are flirting with third-party presidential candidates instead of getting behind Clinton, with a series of new polls showing upward of 40 percent of young voters planning to back Libertarian Gary Johnson or Green Party candidate Jill Stein.

Both the press and Clinton’s campaign have started to pay attention. The Atlantic recently said that millennials “may cost” Clinton the election, while the Daily Beast argued that we should “blame millennials for President Trump.” Mother Jones’ Kevin Drum blames Bernie Sanders for the alarming trend of young people turning against the Democratic Party, while Mother Jones’ editor Clara Jeffery admitted to hating young people on Twitter:

i have never hated millennials more https://t.co/q042TC2Lul pic.twitter.com/zHoqYJ2UwO — Clara Jeffery (@ClaraJeffery) September 15, 2016

This is an alarming, disorienting moment for a party used to being the party of the young. The most immediate danger to them is that young voters’ rising third party interest may give Donald Trump a shot at the White House. But it’s also opening up a series of more emotionally charged questions for the long-term: Weren’t Democrats supposed to be the party of the youth vote? When did they stop winning millennials? And how?

The key to understanding what’s going on here is that Democrats have largely not lost the young voters they once had. Instead, what’s mostly happened is that America has now spawned a different and newer mutation of the millennial strain — one formed in its own culture and with its own political genes. Whether this new type of millennial will follow its peers into the Democratic Party may be one of the most important political questions in the election, but — aside from largely despising Trump — nobody knows exactly where they’ll end up. And that’s giving third parties an opening.

The old millennials versus young millennials “schism”: the key to understanding youth politics

The first step to understanding youth voting patterns is to break the youth vote into two separate groups — the “old millennials” and the “young millennials.”

These two groups have their own distinct political identities that often gets glossed over in media coverage, according to John Della Volpe, a leading researcher of millennial voting patterns at Harvard’s Institute of Politics.

But Della Volpe says there really is a measurable “millennial schism.” He suggests we can even put an approximate date on it: Young millennials were born after November 4, 1990, and old millennials were born before that date. (Preemptive apologies here if this makes those reading this feel old.)

This day isn’t chosen at random: It’s the cut-off point for being old enough to vote for President Obama in 2008. The millennials on one side of this divide — now aged about 26 to 35 — have become more reliable Democratic allies. Polling that zeroes in on this age group is scarce, but what we have suggests Clinton will secure close to 50 percent of these older millennials. “Older millennials are supporting Clinton at about the rate you'd expect,” says Drum, of Mother Jones.

Then there’s a separate group of younger millennials — those now aged 18 to 25. They do not have the same allegiance to the Democratic Party, and their political identity is more fluid, Della Volpe says. Depending on the pollster, this group’s support for Clinton hovers closer to the 35 percent mark when third parties are included.

This intragenerational divide has become increasingly clear over the course of the presidential election. Looking at data from the firm SurveyMonkey last week, FiveThirtyEight’s Harry Enten says that 25- to 35-year-olds prefer Clinton by a 22-point margin. (That’s the same as Obama’s 22-point margin of victory among millennials in 2012.) By comparison, Clinton holds just a 13-point advantage for among those aged 18 to 24, according to Enten.

That may not sound too dramatic, and we should be careful not to overstate it. But the divide between young and old millennials over Clinton is about as great as it is between millennials overall and voters in their 40s. And that, in turn, is a good hint of why it’s so incoherent to lump young and old millennials together as one uniform voting bloc.

Young millennials are much more skeptical of the Democratic Party than old millennials

It’s become something of a truism in the media that young voters are much more likely to be Democratic. And that does remain true — at least until you get to around the 26-year-old mark.

Go any younger, however, and the trend begins to reverse itself.

“To me, the most interesting thing about this young-old millennial divide is that the younger millennials are less likely to be Democratic,” Della Volpe says.

This is not the same as saying that the younger millennials are less “liberal.” Instead, the younger millennials appear to be somewhat less loyal to the Democratic Party — a divide that first became clear nationally during the presidential primary, when Bernie Sanders secured a massive 81 percent of those aged 18 to 24. (Clinton consistently performed about 20 points better among old millennials than young millennials throughout various stages of the primary.)

But Della Volpe argues we had evidence of a growing divide even beforehand. He points to earlier signs that younger millennials were less tied to the Democratic Party than their older peers:

In Wisconsin in 2013, Republican Gov. Scott Walker won 18- to 24-year-olds “outright,” according to Della Volpe. He lost among voters aged 24 to 29 years old.

In Virginia in 13, Republican Gov. Ken Cuccinelli won those aged 18 to 24 by 6 points, while voters aged 25 to 29 went to Democratic Gov. Terry McAuliffe by a 15-point margin.

Younger millennials have been less likely to register as Democrats than older millennials. In 2014, they said they were more open to supporting Republican presidential candidates than older millennials.

Young millennials have consistently different policy views than older millennials, and are in particularly more conservative on fiscal issues. “They aren’t huge swings, but there still are significant differences,” Della Volpe says.

Some observers have been perplexed that the same young voters who so clearly thrilled to Sanders are now flocking to Libertarian candidate Gary Johnson, who is clearly to the right on economic issues of Hillary Clinton.

But this confusion dissipates once you begin seeing young millennials not as strict socialists but as Democratic Party skeptics. Indeed, Boston College political scientist David Hopkins has argued that Sanders excelled because he ran in the primary as an “outsider” candidate — not because the primary represented any specific policy referendum.

This doesn’t mean Sanders’s youth base was imaginary, or that they didn’t support him because his policy ideas powerfully resonated with them. But it does reinforce researchers’ suspicions that 18- to 25-year-olds are more politically adrift, seeking a movement or party to grab onto. And for now, at least, Hillary Clinton is not their lifeline.

One theory of what’s driving the old millennial vs. young millennial split

The way we name our generations is a bit murky, a combination of what makes sense to academics and what catches fire in the press. The term “millennials,” for one, was coined for a book published in 1987 — before many millennials were even born.

But there’s no reason these generational titles are particularly useful for understanding voting behavior — or that they automatically map onto the lived political experience of Americans.

Unsurprisingly, it turns out they don’t. Della Volpe says that his research points to one obvious reason for this new generational divide among millennials: Younger millennials didn’t develop a “political consciousness” until several years after George W. Bush was out of office.

Political scientists have identified a phenomenon called “generational imprinting” — the idea that attachments to parties formed during the late teens turn out to prove remarkably durable for life. This crucial and formative moment for many older millennials was the mid-to-late 2000s — the George W. Bush presidency and the ascendancy of Barack Obama — which is a distant memory for much of the very youngest cohort in American politics today.

During that time period, Della Volpe says, older millennials were hit by a “perfect storm” of news events to become Democratic Party allies. The Iraq War, the Bush administration’s handling of Hurricane Katrina, the prominent role young people played in powering Obama’s 2008 presidential campaign — all helped make them the most reliably Democratic generation in decades.

By contrast, young millennials were often in middle school or early in high school for these events — too young for them to have been foundational political experiences. Today’s 18-year-old voters would have been 10 when Bush left office. Their “generational imprinting” process is still being formed, but so far young millennials have generally come of age to see a Washington, DC, riven by gridlock neither party seems equipped to solve, he says.

“These are the folks who weren’t part of the ‘Obama movement,’ and they never developed that distaste for Bush that many of the older millennials did,” Della Volpe says.

Viewed one way, this makes young millennials less aware of the dangers posed by a Republican administration than older millennials. Viewed another, they’ve developed a clear-eyed understanding that neither major party is equipped to tackle the big problems in American politics.

Where do the young political nomads go?

Della Volpe thinks the GOP could have reversed its long slide with young voters this year had it not nominated someone like Donald Trump, a candidate whose views on race and reputation for xenophobia are toxic to young voters.

“This election of 2016 was an incredible opportunity for Republicans to create a relationship with the younger millennials,” Della Volpe. “Trump made sure that relationship was never consummated.”

Where these political nomads find a home instead looks like anyone’s guess. For now, Bernie Sanders is doing his best to make them Democrats. But as the Washington Post’s David Weigel makes clear in a dispatch from the campaign trail, it’s not clear Sanders will be as successful a surrogate as he was as a candidate — only a few hundred people have been attending some of the Vermont senator’s rallies in battleground states, for instance.

“I don’t think anybody knows whether this is going to work,” Della Volpe says.

So where will the young millennials go? Most stories suggest that they’ll come into the Democratic or Republican folds. But Gary Jacobson, a political scientist who studies generational imprinting at the University of California San Diego, thinks there really might be an opening for a third party.

Jacobson suggests imagining a Clinton presidency that suffers a high-profile disaster — another recession, a disastrous war, a national security emergency — as the GOP hardens its Trumpian rhetoric on immigration and Muslims. That might make both major parties unpalatable to young millennials.

By the time 2020 rolls around, the question about the youngest voters might not be whether the youngest voters will vote for a third party. It’ll be which one they support.