A new poll from Suffolk University and USA Today, though, suggests a big shift among African Americans that could undermine that confidence. We have to note up front that this is one poll, with large margins of error on black responses. That said, the shift is surprising.
Since the last Suffolk poll in July, the overall race has tightened. Clinton still leads, but by less than she used to. Now notice how big the drop among black Democrats is on the chart below.
We've added bars that show the 10 percent margin of error for the group, showing how much variability is at play. But even so, it's a giant drop-off. Among white Democrats, Clinton is doing nine points worse since July. With black Democrats, this poll has her down 31.
The number of undecided voters doubled between the two polls, which is also noteworthy. Voters who are changing their minds about whom to support often make a waystation in the "undecided" category rather than switching immediately from one candidate to the next.
Part of the shift in support for Clinton might be due to the decline in her net favorability among blacks. Opinions of Clinton have dropped among all Democrats as we've seen before. Among blacks, the decline has been greater -- and the increase in favorable views of Sanders has been stark. Clinton is down 16 points in net favorability, and Sanders is up 31. (The margin of error here, we would reinforce, is 8.5 points.)
(Sanders also saw a big decline in the number of voters who hadn't heard of him. In July, 23 percent of Democrats and 35 percent of black voters were unfamiliar with him. Now the percentages are 5 for both.)
Clinton still leads, of course, and Joe Biden isn't in the race. The vice president does better with blacks than Sanders, and past polls have shown that many Biden backers would go to Clinton if he were to decide not to run. A quarter of those who had a preferred candidate said Clinton would be their No. 2, and 31 percent said theirs would be Biden. But 20 percent said they would go to Sanders before either Biden or Clinton.
It's also important to note that this new poll doesn't show much change in support for the top three Democrats over other recent polls. Clinton's been at or a bit above 40 percent for a while, while Sanders has been in the mid-20s.
It's that shift among blacks that's most noteworthy. One poll. Large margin of error. But if it turns out to be predictive, it could significantly shift the Democratic contest.