A new poll shows that Joe Biden lost support but extended his lead after the Democratic debates—mostly at the expense of Vermont Sen. Bernie Sanders, who plummeted since a pre-debate survey in June.

Despite losing 4 points of his support from 34 percent to 30, Biden extended his lead in the Democratic primary from 7 points in June to 15, as Sanders dropped 12 points to 15 percent from his June number of 27 percent, according to the Emerson Polling survey released Wednesday.

The poll also showed that California Sen. Kamala Harris doubled her support while Massachusetts Sen. Elizabeth Warren held steady.

Harris’ numbers improved 8 points to 15 percent, and Warren ticked up a point from June to 15 percent, creating a three-way tie for second place behind the ­former vice president.

Thirty percent of voters said Harris had the best performance in the Democratic debates, while 18 percent said Biden performed the best, 13 percent said Warren and 10 percent chose Sanders.

But another 19 percent said Biden — who stumbled when confronted by Harris for comments on racial issues — performed the worst.

Spencer Kimball, director of Emerson Polling, said “these numbers suggest that the Democratic field is still fluid and that Biden will need to improve upon his performance to maintain his lead.”

Biden’s lead held up in all age groups, a departure from earlier Emerson polls that showed Sanders consistently leading among the youngest group of voters.

Among 18- to 29-year-olds, 26 percent backed Biden, 24 supported Sanders and 10 went for Warren.

Among 30- to 49-year-olds, Biden led with 23 percent followed by Sanders with 19 and Harris and Warren with 16 each.

With voters aged between 50 and 64, Biden led with 36 percent followed by Harris with 22, Warren with 15 and Sanders with 8.

For those over the age of 65, Biden received 38 percent, followed by Warren with 19, Harris with 14, and Sanders with 8.

Since the June Emerson poll, President Trump has seen his head-to-head numbers improve against all his Democratic rivals.

Trump now leads Warren, South Bend, Ind. Mayor Pete Buttigieg, and Harris by 51 percent to 49 percent each after trailing them all last month.

Biden leads Trump 53-47, and Sanders leads Trump 51-49.

The most important issue for voters overall in deciding their presidential pick is the economy at 26 percent, followed by health care at 21, immigration at 17, and social issues at 16.