And what is heard in four new radio ads, two for Republican mayoral candidate Sam Katz and two for Democrat John F. Street, is that each believes one issue above all others will sway undecided voters.

That issue: education.

Street's two new ads talk about nothing else, and Katz devotes one of his ads to the subject. The other Katz spot is a mixture of negative comments about Street and positive reasons for supporting Katz.

In fact, that ad is the only of the four that strays from the negative. The other Katz ad mentions Street's name six times, all derisively, twice as often as Katz's. The first line sets the tone: "You've got to wonder what John Street is thinking about."

Neither Street ad ever gives one reason to vote for Street, only reasons to vote against Katz.

Why such a preponderance of negative comments?

"Toward the end of a campaign, when you switch to positive on television, you want to keep a foot on the other guy's neck," said one campaign insider. "Radio and direct mail are their ways to do it."

The ads do differ noticeably in one way: style. Both Street spots have a touch of humor; the Katz ads are both solemn. This could be a reflection of the men making the ads. David Axelrod, the media consultant for Street, tends to be a lot less somber than Katz's media man, Neil Oxman.

So one Street ad attacking Katz for his poor attendance record when he was on the school board from 1981 to 1984 starts with a female teacher calling the roll in a classroom: "Billy. Here. Molly. Here."

Then it switches to a male voice: "The first thing we expect of our children is that they show up for class. But what about the adults we entrust with their future?"

This is followed by the sound of a gavel striking for order and another roll call: "Mr. Thomas. Present. Mr. Jones. Present. Mr. Katz (silence). Mr. Katz (silence)."

And then the final shot: "During his brief service on the Philadelphia school board in the early '80s, Sam Katz amassed the worst attendance record of any member."

That is true, although a story in The Inquirer on Oct. 13 that recounted Katz's record pointed out that colleagues on the board said he was helpful in solving a financial crisis in the school system - and that Katz said he stayed in close touch with board matters while on the road on business.

In the department of leaving things unsaid, one of the Katz ads ridicules Street for a proposal to give state government more authority over Philadelphia schools in return for more state money.

"First, Street says he'll turn the public schools over to a state board. But the idea gets resoundingly criticized . . ."

Not mentioned is that the lead critic was Gov. Ridge. The reason for the omission: Katz, trying to get as many Democratic votes as possible, wants to keep as much distance as possible between himself and the Republican Party.

Street, in one of his ads, which has a Halloween theme, is there to say: "Sam Katz has embraced the party and the program of Gingrich, Ridge and Santorum. Republican Sam Katz. All trick and no treat."

Besides being negative, neither candidate is reluctant to exaggerate.

Just as Street stretched the point in suggesting that Katz has fallen in line behind former House Speaker Newt Gingrich, it was a substantial stretch for Katz to to say that Street wanted to turn the schools over to the state. What Street proposed was letting the state exercise greater oversight over school spending - so that legislative leaders would feel comfortable approving more state aid for the Philadelphia School District.

One of the Katz radio ads, bringing negative ads to their logical conclusion, attacks Street for his negative ads.

"Only days to go in the campaign, and instead of proposing solutions, all John Street continues to do is be negative and attack Sam Katz," states one Katz ad.

It ends: "We want more from the next mayor of Philadelphia than the negative campaign of John Street."