Or, anyway, Clinton had never trailed in a major poll when NBC released its survey. An hour later, Fox News dropped a bombshell of its own: Sanders has a 3-point lead, according to its new national survey.

A year ago, Clinton had a 52-point lead over Sanders, with 55 percent of support to his 3 percent. In Fox's last poll, conducted shortly before Iowa, she still led by 12. No longer.

At least according to this one poll. This poll is pretty remarkable in that every demographic has shifted in Sanders's favor, with him increasing his support and Clinton losing hers. In every single one. Where Clinton used to lead big, now the two are often tied. Where they were basically tied, Sanders has surged forward.

Certainly possible. But it's worth being cautious. In the words of Nate Silver:

We're not going to ignore this one, but we'll try not to obsess over it.

Fox also released a new survey from South Carolina today. In it, in a state where Hillary Clinton and Sanders have both been campaigning, the numbers are very different -- including among white voters.

South Carolina is not as liberal a state as Vermont, even among Democrats, but lots of other states aren't as liberal as Vermont, either.

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As we noted about NBC's weird poll on Wednesday, this could perhaps be a signal of a big shift in the race. Or it could be a poll that's exploring the limits of the margin of error. As Silver points out, the average is the better indicator, and Clinton still leads in Real Clear Politics' average.

Though that Fox poll certainly did some damage.

* The NBC/Journal poll isn't the point of this article, but I did want to note this interesting question from it. Asked what concerned them most about Hillary Clinton, a fifth of Democrats said her ties to Wall Street. Two-thirds said nothing. Only 7 percent said it was the email server!