After June's Democratic presidential debates, Sen. Kamala Harris has gone from a high of 20% of Democratic voters who favor her to just 7% on Tuesday, per Quinnipiac.

Why it matters: That 13-point drop in just one month indicates Harris' post-debate "sugar-high" might not be aging well, despite the fact that she's firmly established herself in the top tier of candidates in several national 2020 polls.

Meanwhile, Joe Biden — a frequent target of Harris' attacks — has gone from a low of 22% after their debate brawl and has inched back up to 32%.

He fluctuated between 29% and 38% between March and June.

Between the lines: 14% of Democratic voters said they would "absolutely not vote for" Biden if the primaries were held today — the only candidate to earn double-digit percentage points on that question, and twice as much as the 7% who said the same of Harris, per a Politico/Morning Consult poll.

By the numbers: 43% of Democratic primary voters said Harris' performance was "excellent" after the first round of debates in June, according to Politico/Morning Consult.

That fell to just 20% of Democratic voters who said the same of her Detroit debate performance in July.

Only 9% of Democratic primary voters prefer Harris, per a Morning Consult poll out this week — down 3 percentage points from their pre-debate polling.

Her net favorability has dropped by 11 points, which Morning Consult notes is 7 points more than any other 2020 Democrat.

What they're saying: "One reason for her fall: I don’t think anybody can figure out what her message is. We know what Warren’s message is, and Bernie’s message, and we kinda know Biden's message even if he’s not delivering it very well," said a Democratic pollster who's talked with several 2020 campaigns. "But who is she really? And what is her message?"

Other pollsters, who asked to speak candidly because their firms are aligned with various candidates, described it as a "sugar-high" right after the first debate that now might not have aged as well since.

Harris' campaign declined to comment.

The bottom line: Harris' numbers certainly aren't the end of the world — and she's positioned herself among the top five candidates. But as one Democratic pollster put it: "None of this has to be fatal, but she’s got to get out of this rut."

Go deeper: How Kamala Harris' Medicare for All plan differs from Bernie Sanders