A large bloc of undecided voters in the New York City Democratic mayoral primary could be a critical factor in deciding whether leader Bill de Blasio can avoid a runoff, according to the newest Quinnipiac University poll.

De Blasio leads among likely Democratic primary voters with 39 percent, followed by 25 percent for William Thompson, the poll, released Monday, shows.

Less than a week ago -- Tuesday -- a poll of voters showed de Blasio with a 43 percent; a margin sufficient for him to narrowly pass the 40 percent threshold needed to avoid a runoff.

"It looks as if Public Advocate Bill de Blasio couldn't hold that 43 percent in a week when he was in the spotlight and he got walloped by everybody," said Maurice Carroll, director of the Quinnipiac University Polling Institute.

"His support by black voters slipped just enough to make a runoff possible. But he's ever so close."

Sunday's poll showed 18 percent for City Council Speaker Christine Quinn, 6 percent for former Rep. Anthony Weiner, 4 percent for Comptroller John Liu, 1 percent for former Council member Sal Albanese and 8 percent undecided.

advertisement | advertise on newsday

The previous survey by the independent Quinnipiac institute had Thompson with 20 percent and Quinn with 18 percent.

Carroll said the undecided voters mean a great deal to de Blasio and the chances for a runoff. "If de Blasio picks up just a few of those undecided voters, he's over the top," Carroll said. "In our last few days of polling, however, we're seeing the movement to 2009 Democratic nominee William Thompson.

Sunday's survey also showed that among Democratic primary voters who do name a candidate, 18 percent said there is still a "good chance" they will change their mind by Tuesday's primary.

The survey also showed De Blasio ahead with black voters at 37 percent, followed by Thompson with 27 percent and Quinn with 9 percent. White voters go 40 percent for de Blasio, 26 percent for Quinn and 24 percent for Thompson. Hispanic voters go 44 percent for de Blasio, 26 percent for Thompson and 14 percent for Quinn.

Women back de Blasio with 40 percent, followed by 22 percent for Thompson and 19 percent for Quinn. Men go 38 percent for de Blasio, 28 percent for Thompson and 16 percent for Quinn.

In the comptroller' race, Manhattan Borough President Scott Stringer broke out of the too-close-to-call event with 50 percent of likely Democratic primary voters, the poll said.

Former Gov. Eliot Spitzer had 43 percent, with another 7 percent undecided. Among those who named a candidate, 13 percent said there is a "good chance" they will change their mind by Tuesday.

The previous poll showed Stringer with 47 percent, Spitzer with 45 percent.