But each candidate seems to be pursuing a different strategy, a difference that goes to the heart of what Katz and Street are trying to accomplish at this stage of the campaign.

In Street's ads, real people - including the candidate - do the talking. In Katz's spots, a ubiquitous voice-over does the speaking.

Street has put four ads on television since Labor Day. In two of them, he does the talking about his hopes for the city and his plan for improving education. In a third, the talking is done by his fellow Democrat and longtime political collaborator, Mayor Rendell, who calls Street "one of the great healers in municipal government anywhere."

Only in an ad ridiculing Katz for urging school tax cuts when he ran for governor five years ago - a position Katz's campaign director Bob Barnett calls "irrelevant" to this campaign - does Street use a voice-over. Seldom in political ads do candidates dare say anything negative in their own voices.

Katz, on the other hand, depends exclusively on the voice-over in his three post-Labor Day ads.

Why the difference?

Essentially, the Katz ads say things that would sound arrogant if they came from the candidate's mouth, while the Street ads are trying to create an impression of the candidate that can be achieved only if he does the talking.

"If you're talking about accomplishments - something the candidate has done - it sounds as though he's bragging if he says it," said David Doak, a Washington political consultant who has worked in Philadelphia elections.

The voice-over, he said, is meant to give the illusion that an independent third party is talking objectively.

The Katz ads boast that Public Financial Management, the financial consulting firm he assisted in leading for 18 years, helped "cities all across the country build schools, improve mass transit, and provide better services to taxpayers."

But more important, Katz seeks to trumpet his stature as a serious candidate. In a city where registered Democrats outnumber Republicans nearly 4-1, Katz must convey the simple notion that he is truly competitive and that a vote for him will be more than a frivolous gesture. And that's what his ads are trying to do - even if they sometimes stretch the truth a bit.

Thus, one ad says that Katz's tough stands on education, crime and jobs "are again, earning the highest marks."

That statement is based on random interviews that the Daily News conducted after a candidate forum in February. The story said, "Katz was mentioned as a top performer by about three-fourths of those interviewed."

A second ad tries to bolster Katz's chances of winning by featuring poll results and a headline in The Inquirer saying that the race appears close.

And in his most recent ad, the voice-over intones: "Thousands of Democrats throughout the city are supporting Sam Katz for mayor. Led by former mayoral candidate John White Jr., Democrats and Republicans are coming together."

Throughout the 30-second spot are shots of Katz talking to blacks and whites who appear to be working people.

If Katz has to establish that he must be taken seriously, Street has a different problem.

He must move past the perception - compounded by the strident attack ads of rival candidate Marty Weinberg during last spring's Democratic primary campaign - that he can be a bully and cares less about white Philadelphians than black ones.

The reigning political wisdom is that Street cannot win unless white liberals are comfortable about voting for him. "One way to rebut attack advertising is to give people direct exposure," said Kathleen Hall Jamieson, dean of the University of Pennsylvania's Annenberg School for Communications and an expert on political advertising. "It provides reassurance and a counter-image to the image of attack advertising."

Thus, the ads in which Street does the talking show him chatting with whites and blacks in two settings, one in a classroom and one apparently at a picnic. He smiles a lot and seems sympathetic.

In the ad that features Rendell, the mayor refers to Street as a unifier and a healer who has "empathy for every neighborhood in the city."

These are not words normally used to describe Street. But if he can't persuade whites in Mount Airy, Center City and University City that they are true, he may be in trouble.

One other fact about the ads deserves mention, especially in a city often characterized as racially divided.

The most recent ad for Katz, the white candidate, prominently features an endorsement from a black, White. And a current ad for Street, the black candidate, prominently features an endorsement from a white, Rendell.

Both ads clearly seek to do the same thing: Make those who might be inclined to reject a candidate because of race rethink their positions.

Katz and Street are scheduled to square off in two debates Saturday.

They will debate for an hour Saturday morning at the studios of WPVI-TV (Channel 6) on City Avenue. The debate, sponsored by the NAACP and the TV station, will be taped for broadcast at 5 p.m. Sunday on Channel 6.

On Saturday afternoon, Street and Katz will meet again at the Zellerbach Theater on the University of Pennsylvania campus. Segments of that meeting, sponsored by the Philadelphia Compact and The Inquirer's Citizen Voices Project, will be aired during the week on radio station WHYY-FM (90.9).