The Democratic candidates for New York City mayor will square off for their final debate exactly one week before the primary.

The unpredictable campaign, which has featured three different front-runners in as many months, will go into the home stretch with a clear dynamic: Public Advocate Bill de Blasio is ahead, and the other candidates are fighting for the second spot in a likely runoff.

Recent polls show de Blasio near the 40 percent mark that would allow him to clinch the nomination outright during the Sept. 10 primary. But if no one crosses that threshold, the top two finishers automatically advance to a runoff three weeks later.

City Council Speaker Christine Quinn and ex-comptroller Bill Thompson are within a point of each other for that second slot, according to recent polls. They are both far behind de Blasio, yet they have trained the majority of their recent attacks — including at the last debate — on him and not on each other.

They will be joined on stage by former congressman Anthony Weiner and City Comptroller John Liu, who each have seen their candidacies dogged by scandal.

Weiner led the race for several weeks in June until his support collapsed in the wake of his latest scandal. Two of Liu’s staffers were convicted of misusing campaign funds though the candidate was never charged with wrongdoing.

Perhaps fitting for one of the most contentious campaigns the city has seen in decades, the format of the final debate itself has become a source of controversy.

The first hour of the 90-minute debate, which was organized by the city’s Campaign Finance Board, will be broadcast on the local NBC affiliate, while the last half-hour will only be shown online. The Quinn, Thompson, Liu and Weiner campaigns signed a letter over the weekend asking that NBC show the entire 90 minutes. The de Blasio campaign did not.

A spokesman for the Campaign Finance Board said the debate, which is co-sponsored by the Wall Street Journal and Telemundo, is only required to be 60 minutes and NBC was “going above and beyond” by offering additional content online. No change was planned.

The general election is Nov. 5.

(AP)