Part of that declining support has also been attributed to shifts in how voters view Clinton, to which there's some truth. On the question of trust and its causes, the picture is a bit more interesting.
Thursday's new Quinnipiac poll offered the fourth version of a question that it has asked since April, about a month after the e-mail story first broke. Here's how perceptions of Clinton's trustworthiness have changed.
With the one obvious exception, the trend has been fairly static. Among all respondents, Clinton's viewed as honest by 34 percent of people, within four points of the high. Republican opinion on her trustworthiness is the same as it was in April. The percentage of independents who see her as trustworthy, has dropped -- though not as much as it did among Democrats.
How independents respond on this question is the million-dollar question for Clinton over the long term -- assuming, as we currently should, that she wins the nomination. The Democratic tumble is interesting; the Republican opposition is expected. That partisan split is why President Obama's approval ratings have been unusually steady: Democrats love him, Republicans hate him, and most of the movement is among declared independents.
One of the issues that might cause someone to question Clinton's honesty is her use of a private e-mail server while serving as secretary of state. Over the course of the e-mail controversy, several pollsters have asked questions about it -- and in many cases, as with the trustworthiness question above, independents' attitudes are closer to the Republican view than the Democratic one.
So here's what we know: Attitudes on Clinton and trust shifted dramatically -- but only in this most recent poll. Independent voters seem to mirror Republican views more closely than do Democrats, including on the e-mail issue.
Meaning one of three this is happening. One, the new poll is an outlier. Two, something fairly recent caused peoples' confidence on Clinton to shift. Or, three, this is intertwined with the growth among other Democratic candidates.
A point in favor of that last option. As we've noted, most independents tend to align with one party or the other over the long run. Meaning that the drop in independent support in the August poll may be a function of Democratic-leaning independents echoing Democrats on the whole. As Sanders and Biden have gained support people may be more willing to be skeptical of Clinton in general.
This is a very good instance in which saying "early polls don't tell us much" is probably right on. One poll, out in the cold lonely woods by itself, doesn't tell us much about why attitudes on Clinton have changed. Here's what we can say: Hillary Clinton's lead over her competitors is not what it once was.