We begin, as one should, with two important caveats.

1. This is one poll.

2. The primaries are still six months away.

But if you are former vice president Joe Biden, Quinnipiac’s new poll of the 2020 Democratic presidential nominating contest is very much not what you want to see — six months out or not. If you’re Sen. Kamala D. Harris (D-Calif.), though? Time for a toast.

The easiest way to explain why is simply to walk through the numbers. The graphs below show the change in support for the four leading candidates among certain demographic groups since Quinnipiac’s April poll. We’ve flagged certain points of data that are worth highlighting.

A. Biden’s lead in April was 26 points over Sen. Elizabeth Warren (D-Mass.). It is now 2 points — within the margin of error — over Harris. That’s particularly galling, no doubt, since it is Harris who offered that devastating question about Biden’s past positions on school segregation in last week’s debate.

B. Harris was the choice of 15 percent of self-identified “very liberal” respondents in the April poll. Now she’s at 21 percent — putting her at the same level of support as Sen. Bernie Sanders (I-Vt.). Warren holds a slight lead.

C. But notice where Harris is with all three ideological groups. Among very liberal voters, 21 percent. Somewhat liberal, 22. Moderate or conservative, 18. The gap between the very liberals and moderates for Warren is 19 points; for Biden, it’s 17 points. In other words, Harris’s support in this poll isn’t contingent on support from one ideological group.

D. Harris’ support among women more than doubled since April and she now leads with that group. Biden’s support with women plunged 17 points.

E. While Harris, Biden and Warren all enjoy about the same level of support from whites (thanks to Biden’s support falling 14 points since April) . ..

F . . . . Harris and Biden stand apart among black respondents.

This is no small thing. Biden’s support from black voters has been a central part of his overall lead. But the 31-point lead he enjoyed over Warren in April is now a 4-point lead over Harris.

Remarkably, there may be an even bigger problem for Biden contained in another question in the poll: Who respondents saw as the candidate with the best chance of beating President Trump.

We only need one marker here.

A. In April, the last time Quinnipiac asked this question, Biden had a 44-point advantage on the electability question. Since then, the percentage of people citing him as most electable has dropped 14 points and his lead over Harris is now only 28 points.

Across demographic groups, the sense that he’s most electable eroded. Down 22 points among somewhat liberal respondents (thanks in part to Harris’s surge), down 15 points with women, down 13 points with whites. The slide was more modest among black respondents, no doubt to Biden’s relief.

This is critically important to Biden, though. If Democrats start to think that he’s not the candidate best able to oust Trump (and a HuffPost-YouGov poll with a different methodology shows that this advantage has already diminished), the foundation of his candidacy is shaken. And Biden, who launched his campaign by taking the fight to Trump last week, lost a debate-stage fight with Harris quite badly.

One poll! Six months out! But a raft of bad news for the man who hoped to have a much easier path to the nomination.