The new poll, released a day before the second round of Democratic primary debates in Detroit, shows Massachusetts Sen. Elizabeth Warren follows Biden with 15% support, California Sen. Kamala Harris with 12%, Vermont Sen. Bernie Sanders at 11%, and South Bend, Indiana, Mayor Pete Buttigieg at 6%. The difference between Warren, Sanders and Harris is within the poll's margin of error, while Biden leads the group.

The biggest mover since the last Quinnipiac poll is Harris, who dropped 8 percentage points in her support in a poll taken in the wake of the first debate. Before that debate, Quinnipiac had Biden at 30% and Harris at 7% -- and immediately after Biden was at 22% and Harris at 20%.

This poll finds much different numbers than the previous survey, with Harris losing some (but not all) of the support she saw in her surge following the first round of debates. Sanders also lost 8 percentage points since June, having gotten no bump from debates. Warren maintained her 15% support; she's had between 12% and 15% support since April.

Polls taken directly after debates tend to show a surge in support for some candidates who become more prominent in the minds of voters after the performance. Harris made waves at the second night of the first debates, calling out Biden on his past policy choices.

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