Sen. Elizabeth Warren (D-Mass.) had a bad night in New Hampshire. She came in fourth after Sen. Bernie Sanders (I-Vt.), former South Bend Mayor Pete Buttigieg, and Sen. Amy Klobuchar (D-Minn.), securing zero delegates in the process. But polling averages over the last three months consistently indicated Warren's New Hampshire base was bigger than it proved: In November, she briefly led the field with 22 percent, and as recently as mid-January she was a strong third with nearly double the backing she got at the ballot box.

If those voters decided not to turn out for Warren — perhaps buying the narrative that her star is fading — why didn't they show up for Sanders, whom conventional wisdom had said would be their natural back-up plan? Sanders got around 26 percent in Iowa, where Warren pulled 18 percent, and again 26 percent in New Hampshire, where Warren had just 9 percent. Even allowing for differences in the two electorates, why didn't he benefit more from her slump?

The question becomes especially pressing as FiveThirtyEight's second-most-likely outcome of the Democratic race (after Sanders winning it outright) is now a brokered convention in which Warren supporters could be too few to grab the nomination for themselves but enough to play kingmaker. Here are a few possible explanations.

Legitimate policy differences

Sanders and Warren are often paired as the "progressive" or "radical" wing of the Democratic frontrunners in contrast with the moderate lane of Klobuchar, Buttigieg, former Vice President Joe Biden, and former New York City Mayor Mike Bloomberg. That grouping isn't unreasonable, but it is an oversimplification. There's daylight between the two.

On economics, Sanders has argued that though they agree on many points of practical policy, Warren is "a capitalist through her bones," and he is not. The "bones" line comes from Warren herself, and while the distinction may seem like semantics from a certain political distance, up close (as in this analysis from Jacobin), it's real.

On foreign policy too, the similarities are obvious — but there are differences which will matter to the voters for whom this is a priority. Likewise, some social conservatives like The New York Times' Ross Douthat and The American Conservative's Rod Dreher have drawn a class war vs. culture war distinction between Sanders and Warren, respectively. They prefer the former; Warren voters probably wouldn't.

Candidate demographics

The "most optimistic story" of Sanders not picking up more erstwhile Warren votes in New Hampshire, posited The Washington Post's Megan McArdle, "is that all the 'I'm voting for a woman come hell or high water' voters went to Klobuchar, and the rest will go to Sanders if she withdraws." McArdle added that she isn't convinced by her own suggestion, but she isn't the only one to raise the possibility.

There aren't a lot of pollsters asking voters whether candidate demographics matter more to them than policy positions, but we do know that among Democratic primary voters who think gender makes a difference for electability, a majority believe being a woman is an advantage. It's plausible that those voters would move to Klobuchar if they thought Warren was done.

However, even if this explanation is correct, Warren obviously also has ideological supporters. If she dropped out, we could still expect many of those voters to choose Sanders if the remaining alternatives are all substantially more centrist — and, indeed, Morning Consult's latest national survey of Democratic primary voters shows Sanders is the second choice of a plurality of Warren voters.

Electability assessments

For all their commonalities, Sanders and Warren have taken very different approaches to the electability question, which is omnipresent in a race to challenge President Trump.

Sanders has leaned into the "revolution" rhetoric and openly claimed the socialist label, betting that America wants a big change. Warren has sought to cast herself as a progressive technocrat, trying to combine big ideas with deliverability. Warren voters drawn to her practical wonk side may believe Sanders is too extreme to win the general election and thus that their second-choice vote should go to a safer, if less idealistic, option.

Following Warren's lead

In her speech in New Hampshire Tuesday night, Warren gave her heartiest congratulations to Klobuchar while implying Sanders and Buttigieg are dividing the Democratic Party and encouraging their supporters to engage in harmful factionalism. "Sen. Sanders and Mayor Buttigieg are both great people and either of them would be a far better president than Donald Trump," she said. "I respect them both. But the fight between factions in our party has taken a sharp turn in recent weeks," she continued, accusing her two rivals of being willing to leave "our party and our politics worse off than how you found it."

Many Warren supporters would likely follow their candidate's lead were she to drop out now and back Sanders. They could be following her in this combative stance, too, deciding to stay home rather than give him their votes.

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