The decline in her polling numbers has been driven by statistically significant drops in support among almost every demographic group surveyed — and, most notably, among the highest-educated voters, the oldest voters and voters who identify as liberal and very liberal — compared to her vote share in an Oct. 7-13 poll conducted before last month’s debate. Warren lost her lead with liberal voters and now trails Sanders by 4 points, 16 percent to 20 percent.

The point at which her support among key groups began to fall matches the beginning of a flashpoint for her candidacy over health care, a top issue for Democratic voters. In the week following the Oct. 15 debate, her first-choice share among the most politically engaged voters fell 2 points, to 26 percent. In the October debate, South Bend Mayor Pete Buttigieg of Indiana criticized Warren for refusing to say whether taxes would go up for middle-class Americans under her “Medicare for All” plan.

Three weeks later, Warren released her proposal to pay for the expansion of the federally funded health care coverage — but in the weeks since, her first-choice support has continued to fall.