Sen. Kamala Harris didn’t break through in the second set of Democratic debates the way she did in the first, and it may have cost her.

Harris suffered an average 3 percentage point decline in five post-debate polls when compared with her pre-debate numbers. When the data site FiveThirtyEight on Wednesday weighted those five polls based on their sample size and quality, Harris suffered the greatest hit of any candidate — 2.8 percentage points.

Front-runner Joe Biden had the second-worst post-debate decline at 1.9 points. The two biggest gainers are competing for some of the same liberal voters: Bernie Sanders (plus-1.8 points) and Elizabeth Warren (plus-1.6).

Biden still owns a comfortable lead using that same metric, leading Sanders 28.4 percent to 17.1 percent. Harris, however, has fallen to single digits at 7.9 percent – in fourth place and well behind third-place Warren (14.6).

There’s no way to prove why Harris fell, but the former California attorney general did draw critical reviews for her defense of her health care plan in the second debate. That was in contrast with her viral confrontation with Biden over racial issues in the first debate, which led to her seeing a large bounce in her polling into double digits.

FiveThirtyEight editor Nate Silver wrote Wednesday that Harris may be stuck between Biden’s more moderate base and the more liberal base shared by Warren and Sanders: “If you look at the Quinnipiac poll, for instance, there’s no single group of Democrats — say, wealthy or young or black Democrats — among whom Harris is polling at any higher than 10 percent, whereas Biden, Warren and Sanders all have fairly distinctive bases."

Harris’ fall was particularly dramatic in the Quinnipiac University poll, where her support fell from 20 percent immediately after the first debate to 7 percent in the poll released Tuesday.

With the most recent data, Silver downgraded Harris in his presidential candidate tiers, to 1c with Sanders. Biden is alone at 1a, and Warren is alone at 1b.

If there’s a bright side for Harris, she doesn’t have much competition below her – her next closest rival according to FiveThirtyEight’s post-debate tracker is Pete Buttigieg, and he’s six percentage points behind her.

Meanwhile, Harris will air the first TV ads of her campaign Thursday, coinciding with a five-day bus tour of Iowa.

Greg Keraghosian is an SFGATE homepage editor. Contact: greg.keraghosian@sfgate.com