Their words say one thing: That Sam Katz and John Street have sharply different views of how to govern America's fifth-largest city - a battle between a tested government official who says he knows how to make City Hall work and a businessman-outsider who would shake things up.

But their style seems to say something altogether different.

In a seemingly endless series of one-on-one mayoral debates, on TV and before business groups and civic activists, Democrat Street and the GOP's Katz appear quite comfortable with each other.

When they disagree about an issue, it's nothing personal. When prompted by the audience, they joke with each other. When the closing credits roll, they put their heads together and appear to be making pleasant small talk.

Yesterday, as Katz and Street debated for an hour on Channel 6, in a forum sponsored by the Inquirer and held at the University of Pennsylvania, viewers saw this typical response when a citizen panelist said that both candidates had satisfied his question about public schools.

Street smiled broadly and clapped, while Katz said: "Finally!"

On Thursday at a forum at Drexel University, when Street got going on a question about city finances, he started referring repeatedly to "Mr. Katz's wage-tax plan."

"I liked it better when you called me, 'Sam,' " Katz responded to laughter.

With so much at stake, and two weeks to go, are these two guys being just too gosh-darned nice to each other?

The odd thing is that if you're one of the many voters who hasn't seen or read much about the debates, and are only getting your information from the frequent TV and radio spots, you might think the mayoral race is a nasty one.

In the last week, Katz has aired radio ads accusing Street of running a negative campaign, while Street during the weekend launched a new TV ad with some harsh things to say about both Katz's tax and education proposals and his role in seeking state money for a proposed auto-racing track near Reading.

But when the bright lights of TV go on for each debate, the candidates don't say anything mean to each other.

Why not?

Here's what the candidates say about it.

"This is not personal," Street said after yesterday's debate, barely pausing to catch his breath during a 15-minute long soliloquy to reporters about why he thinks Katz's plan to reduce the city wage tax below 4 percent by 2003 is "naive."

"I'm always a friendly, respectful and courteous person," Katz said yesterday. "I don't have to engage in a strategy to be who I am."

But politically, of course, there's a lot more at work here.

Political experts note that Street emerged from last May's bruising five-way Democratic mayoral primary winning kudos for his restrained, high-road manner in responding to sharp personal attacks from rival Marty Weinberg.

They said Street's lack of anger was key to convincing voters - especially white liberals who were crucial then and considered even more important in the race with Katz - that he was no longer the sometimes angry young man who physically clashed with reporters and City Council colleagues.

And consider, pundits add, the challenges facing Katz, seeking to convince those same white progressive swing voters that he is nothing like the last white Republican to challenge a black Democrat - Frank Rizzo facing W. Wilson Goode - but a man of calm and moderation.

In other words, they said, the Katz campaign wants to get out an image of their man as a racial healer that whites and a few blacks won't feel guilty voting for - even as most of Rizzo's former supporters cast ballots for him, too.

Political experts said there are other factors at work as well, not least of which is the type of setting in which they have met.

Consider yesterday's debate at Penn's Zellerbach Theater, before a room of community activists and televised at the early hour of 10 a.m., when many Philadelphians are still drinking their morning coffee or getting ready for church.

"That situation is all about having an issues-based discourse," said Democratic political consultant and TV analyst Larry Ceisler, speaking of the forum being broadcast from the halls of a prestigious Ivy League university. "There's almost like a no-fireworks zone, a 'be-nice' zone."

There's one other group likely to impressed by polite, issues-rooted political discourse. That would be the editorial writers at the Daily News and Inquirer, which both campaigns have been aggressively courting - mainly because their opinions are valued most by those undecided white liberals.