Among Democrats who say they will definitely vote in the primary, Ms. Quinn is backed by 26 percent, followed by 19 percent for Mr. Thompson and 14 percent for Mr. de Blasio. A runoff will be held on Oct. 1 if none of the candidates win support from at least 40 percent.

Ms. Quinn far outpaces her rivals in Manhattan, where she is backed by a third of Democratic voters. And she is supported by nearly 4 in 10 voters with household incomes of $100,000 or more.

She also leads among white voters. But among black voters, 24 percent support Mr. Thompson, 18 percent for Ms. Quinn, 15 percent for Mr. Weiner and 12 percent for Mr. de Blasio.