This new survey has Clinton with a 15-point lead, a swing of 18 points toward Sanders. Support for Clinton has fallen to 52 percent nationally, while support for Sanders has increased to 37. Clinton down 7; Sanders up 11. That's a very big swing over the course of a month, one that could suggest that either December's poll or this one is an outlier – or that we're starting to see the sort of pre-election movement that is not uncommon in a primary season.

It's worth looking at how support among demographic groups shifted over that time period. We've broken out Monmouth's numbers below. Clinton's lead among liberals, moderates, men, women, younger people, older people and whites fell since last month — evaporating entirely among liberals, younger people and whites.

Only among black and Latino Democratic voters did Clinton's lead grow, from a healthy 43 points to a massive 50 points.

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We've talked about this a lot recently. It's a critical split in the Democratic base, one which explains why Sanders and Clinton are running close in Iowa and New Hampshire — two very white states — and why Clinton is up by a wide margin in South Carolina (less white). If Clinton and Sanders are tied among white voters as the primaries progress, her support from non-white voters can make a difference in states where whites aren't an overwhelming majority.

Sanders has room to grow. Clinton is seen more favorably in the new Monmouth poll, but only because one-quarter of respondents still don't know enough about Sanders to have an opinion. Twenty-six percent of the people Monmouth talked to had no opinion of Sanders to 9 percent for Clinton — suggesting that a decent part of the population, including many non-white voters, don't know much about the Vermont senator at all.

That said, there's an important bit of good news for Clinton worth mentioning. While many Democrats think Clinton and Sanders would be equally likely to defeat top Republican contenders, far more people say that only about Clinton. As Democratic voters make up their minds, the ability to defeat Republicans in November will be a critical consideration, and here Clinton has an advantage.