Clinton entered that Democratic primary race as the front-runner with 32 percent of Democratic support that May. Barack Obama was in second with 18 percent.
If you track Clinton's approval ratings in Quinnipiac polling over the course of that campaign, they remained relatively stable.
Between May of 2007 and May of 2008 -- after her aspirations of winning the nomination had vanished and at which point she trailed Obama by four points -- Clinton's net approval rating (positive views minus negative views) among all voters rose slightly, staying at about zero -- a wash. Her approval among independents was relatively flat, while her approval among Democrats plunged, thanks no doubt to the difficult race she fought against her popular competitor.
Compare that to Quinnipiac's polling from this year. Between March 2015 and December, Clinton's net favorability -- those viewing her favorably minus those viewing her unfavorably -- sank from plus-3 among all voters to negative-8, an 11-point change. But among independents, that figure went from +4 to -27, a swing of 31 points.
Clinton's dip among Democrats and Republicans is about the same; among independents, some of whom will vote in the Democratic primaries in the states that allow them to, the dip is far larger. But Clinton does better with moderate voters in most polls, including Quinnipiac's, which is the camp into which one would assume many Democrat-leaning independents would fall. So why the difference?
The Quinnipiac chart from 2007 and 2008 is somewhat incomplete, you'll notice, missing polls from the crucial period during which the Democratic race was all but settled. We can use Fox News polls to fill in the gaps. Looking at Fox's polling from that period, we see a lot more change among independents -- a big bubble of support toward the end of 2007 and a collapse after the voting began.
That movement in 2007 mirrors the transition from Clinton-as-inevitable to Clinton-as-troubled. Or, put more generally: When there was a campaign. Clinton held a big lead in that cycle essentially until Iowa. At that point, Obama gained rapidly.
This year, Clinton's fight began a lot earlier, mirroring the dip in approval. There are other factors, too, like the various controversies that have surrounded her campaign. On the subject of the email server, for example, independents regularly come down about in the middle between Democrats and Republican on how important the issue is. Here's a CBS News survey from October 2015:
In other words, the drop is stark, but it's not clear that it's permanent -- or unrelated to Clinton's withered lead in the Democratic race.
To that point, it doesn't seem to be hurting her in head-to-head polling. Since May, support from independents in a theoretical Clinton-Trump general election has mostly mirrored the overall vote, with an exception in November.
General election polling at this point is essentially useless, and the partisan split on Clinton may be why the overall margin mirrors how independents feel (since Republicans and Democrats aren't going up and down much).
Her overall margin has still been going up since September.