Betting line: If you're wondering why candidates aren't lining up to run against San Francisco Mayor Gavin Newsom in the wake of the big City Hall sex scandal, look no further than a recent poll by Fairbank, Maslin, Maullin and Associates.

The poll of 400 San Francisco voters was commissioned by the Police Officers Association to test the public's appetite for a pension measure that the union is thinking of putting on the ballot.

As is often the case in such surveys, the cops tossed in a few other questions -- including one asking voters how they felt about Newsom.

The poll was taken from Jan. 20-23, a week before news broke of the mayor's affair with a former staffer and wife of his now-former campaign manager. But the results show just how far Newsom might have to fall to be vulnerable this November.

A total of 77 percent of respondents said they approved of the job Newsom was doing and 63 percent said he should be re-elected.

When stacked up against other possible candidates, the results were:

-- Newsom: 58 percent.

-- Former Supervisor Matt Gonzalez: 13 percent.

-- Public Defender Jeff Adachi: 4 percent.

-- Supervisor Ross Mirkarimi: 3 percent.

-- Board of Supervisors President Aaron Peskin: 1 percent.

-- Don't know or don't care: 21 percent.

Plus, the mayor already has raised $600,000, giving him a big advantage over any competition.