Bill de Blasio and family at the West Indian Day parade Monday. (Tina Fineberg/AP) Bill de Blasio and family at the West Indian Day parade Monday. (Tina Fineberg/Associated Press)

Last week’s poll vault by Bill de Blasio in his quest to win the Democratic nomination for mayor of New York City has turned into a catapult. The Quinnipiac Poll released Tuesday puts the city’s public advocate above the 40 percent threshold needed to avoid a runoff. No wonder de Blasio and his resonant biracial family appear to be jumping for joy.

De Blasio jumped from 36 percent support last week to 43 percent now. Former city comptroller Bill Thompson came in second again, but continues to flat line at 20 percent. And the steady decline in support for City Council Speaker Christine Quinn put her support at 18 percent. The former front runner is down 9 points since her high of 27 percent in July 29. But her slide is nowhere near the plummet experienced by Anthony Weiner, who once led Quinn 26 percent to 22 percent in a July 24 poll. The serial sexter now enjoys 7 percent support.

There are two killer numbers in the poll that will snuff out the ambitions of Thompson and Quinn if they repeat themselves at the ballot box on Sept. 10. Quinn is the only woman in the race for the nomination, but she loses that vote to de Blasio (18 percent to his 44 percent). Thompson is the only African American in the race, but he loses that vote to de Blasio (25 percent to his 47 percent). Now, there is no doubt that it helps that the front runner is married to a black woman and has two biracial children, one with a phenomenal Afro that I wish I could grow. But there’s something else at work here.

Thompson is the black candidate without the unflinching support of black voters. The 25 percent support he enjoys in the Q poll is nearly identical to the 23 percent of the black vote he won seeking to keep Bloomberg from a third term in 2009. Thus, Thompson’s failure to capture the black vote says more about him than it does about the attraction to de Blasio’s Benetton-esque family.

Here’s another thing to remember. Thompson came within four points of beating Bloomberg in the last election. There was widespread anger four years ago at the mayor’s (and Quinn’s) change of the term-limits law that made Bloomberg’s extra term possible. Yet, as one New York political friend correctly complained, Thompson did nothing with that remarkable accomplishment and the platform that came with it. He basically disappeared for the last four years.

That Thompson is besting Quinn for the second place spot is great for  him. That de Blasio appears to be taking the nomination without need for a runoff makes Thompson’s standing meaningless.

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