And the reason is pretty simple. New CNN/ORC polling in the two states paints a nearly perfect picture of what we generally understand about the Democratic race.
1. Hillary Clinton's lead has shrunk dramatically
In a three-way race with Sanders and Vice President Biden, Clinton has dropped significantly from the huge lead she saw earlier this year nationally. CNN/ORC puts her only 16 points ahead of Sanders in Nevada -- still a big lead by any normal definition, but not what her lead once was.
2. But things change if you take Biden out of the picture
That's because of Joe Biden. As we've repeatedly noted, Clinton and Biden largely share a base of support. Take him out of the picture -- that is, if he doesn't run -- and Clinton's lead over Sanders expands.
In Nevada, Clinton gains four Biden votes for every one that goes to Sanders -- meaning that if Biden doesn't run, her lead in South Carolina balloons to 50 points, which it's hard to see Sanders overcoming.
3. Opinions among Democrats are splintered on racial lines
Now we focus on South Carolina, which is one poll in one state -- but this is the only good, recent poll in the first state with a significant population of black voters.
And in South Carolina, Clinton leads Sanders by 55 points among blacks -- even with Biden running. She trails him by 13 with whites, though if Biden doesn't run, the two are essentially even there.
So to recap: Sanders is putting up a strong fight against Clinton, but he needs Biden to run in order to split her base of support. And in states with larger percentage of black voters, even that isn't likely to be enough.
Of course, all of this is being said about 36 hours before the first Democratic debate, in which who-knows-what will happen. Maybe dreamy Martin O'Malley steps in and makes things more complicated! Polls are snapshots of a very fluid race. But the picture we're looking at here is pretty precisely what we might expect to see.