What’s going on here? I’m not sure there’s a definitive answer, but here are a few theories.

It’s Natural Tightening

According to this theory, there’s no surprise that things are closer now. Voters are just starting to pay attention, and elections almost always get closer at the end. It happened in 2004, it happened in 2008 (with ultimately disastrous results for Clinton), and it’s happening again. This is the view of the Clinton campaign, which says it’s expected a close race all along. “Since the campaign started, we have said this race will be a competitive, tough race that would tighten and we’d have to earn the nomination,” says spokesman Jesse Ferguson. “We have built a tremendous grassroots organization in Iowa fueled by enthusiasm for Hillary Clinton and her record, that is set to compete and win.”

The Message Is Resonating

The Sanders campaign sees the same dynamic from a different perspective, which also relies on the idea that voters are just tuning in. They say that this is proof that Sanders’s electability and message about inequality are resonating with voters now. Another, less charitable view put forward by Sanders backers is that people are just sick of Clinton—though why would that would manifest itself only now, and not in (say) South Carolina?

The Ad Blitz

Relatedly, Sanders aired a ton of ads starting in December, and that may be pushing his numbers up.

The Polls Are Bad

Here are the last four polls included in RealClearPolitics’ Iowa average, all of them taken this month: Clinton +3; Sanders +3; Clinton +6; Sanders +5. Or take that Monmouth poll in New Hampshire, with the massive lead. The fact is, state-level polls are often unreliable at this stage, especially as they move to determine who’s a “likely voter/caucusgoer.” The Quinnipiac poll, in particular, has gotten some races badly wrong and irks many poll-watchers. But polling in general has been pretty rough recently. Maybe none of this is even happening in real life.

It’s All About Trump

Sanders has sharpened some of his rhetoric about the differences between himself and Clinton over time, but the candidate who’s really out bashing her (and even more pointedly, her husband) is Donald Trump, bringing up some uncomfortable questions about sexual assault and Bill Clinton’s past. A side effect of these attacks is that they remind voters of the ugly mudslinging of the 1990s. Even if they think it’s a Republican witch hunt, do they really want to go through all that again?

Sanders Hasn’t Faced Fierce Attacks

That’s a good reminder that Sanders hasn’t been attacked like Clinton has. He looks electable now because no one’s laying many blows on him—Clinton has jabbed at him on guns, but Martin O’Malley is MIA and the Republicans either don’t see him as a serious threat or are just as happy for Clinton to sweat. Put him through that grinder, and will his electability numbers still look so good?