There he goes, down past the pots and pans in the Value Plus discount store: John F. Street, candidate for mayor, in full campaign form, meeting and greeting Philadelphians and getting cheek to cheek as his "Polaroid Posse" snaps pictures for voters to take home. Street works the whole store: glad-handing in the linen aisle--Snap!--and in front of the Christmas tinsel--Snap!--and over by the briefs and T-shirts. Grabbing men, women, children, he is campaigning harder than many thought he'd have to.

A Democrat in an overwhelmingly Democratic town, a former City Council president who can take some credit, along with outgoing Mayor Edward Rendell (D), for Philadelphia's Center City renaissance, Street ought to be coasting to easy victory. But he is not. Indeed, there may be no victory at all. After a summer surge that caught everyone by surprise, a Republican named Sam Katz could give Philadelphia's City Hall to the GOP for the first time in 52 years.

Katz is white. Street is black. Though both men are running inclusive campaigns with little mention of race, race-based voting traditions die hard here, and analysts are watching closely to see whether race will trump party--as polls increasingly suggest--and allow Katz to siphon off a sizable portion of Philadelphia's usually loyal white Democrats.

The election will test a trend that has seen the relevance of race recede from big-city politics in a series of mayoral and other contests around the country. Most recently it happened in the Baltimore mayoral primary, when the city's predominantly African American Democrats elected a white man last month as their nominee.

In Philadelphia's case, the color combination is different: This predominantly white city is being asked to support a black mayoral candidate, not for the first time, but against a strong, white Republican candidate.

"White Democrats will be decisive," said Thaddeus Mathis, a political analyst and associate dean of the school of social administration at Temple University here. While Mathis and others expect some black Democrats also to defect to Katz because of disaffection with Street's brusque political style, those numbers are expected to be relatively small.

"If Street loses, it'll be another case where whites were unable to vote for a black candidate," Mathis said.

But one of the white Democrats who is giving Katz serious consideration, who agreed to speak about the contest without being named for fear of political repercussions, said people like him are open to Republican overtures because Street "didn't do nothing for us. John Street has not given one hoot" about areas of white working-class Philadelphia. "For a lot of people, it's a racial perception. But it's not. It's that people feel disengaged."

Asked in an interview why Katz has been able to get such traction in the campaign that the two men's polling numbers show a dead heat, Street said, "I think part of it is issues of race." He declined to elaborate.

But Street, who is running a cross-racial campaign and depending on votes wherever he can get them, said, "I think I'm gonna win the election. I think I'm gonna get a lot of votes, white and black."

Katz says much the same thing. He is not focusing his campaign on white Democrats, he said one night after a campaign stop. "I'm getting all Democrats to vote for me."

The race here has assumed heightened importance because of Pennsylvania's status as a swing state in presidential elections. It would be a huge psychological blow, as well, for this town that is 4-to-1 Democratic to go Republican. Democrats are pressing hard to keep Philadelphia in the party fold. James Carville, the renowned Democratic consultant, has campaigned here for Street, as has President Clinton, with Massachusetts Sen. Edward M. Kennedy in tow for a Friday rally with the Democratic candidate.

Strangely, no big-name Republicans have been here. Katz hasn't sought them. Though running as a Republican, he is not touting a party line; to do so could alienate party-conscious Democrats. Rather, Katz is trying to attract voters as a mere Philadelphian with nonpartisan interests. Katz was a Democrat until 1990.

"I need the people in this room who are Democrats to overlook party lines," Katz told a small crowd in the dingy yellow and chartreuse basement meeting room of the Wissinoming Presbyterian Church on a recent night.

Wissinoming, a struggling white working-class community in Northeast Philadelphia that doesn't have a bank branch and lost several homes to strange soil sinkage earlier this year, has a tradition of voting Democratic. But Katz has come here many times during the campaign, for these are the kinds of people most likely to jump and elect a Republican.

"He's playing the community," said Glenn Devitt, chairman of the Wissinoming Civic Association. "Street's not."

Both Street, 56, and Katz, 49, say they want to build on the success of the Rendell years, during which Philadelphia was pulled back from bankruptcy and its downtown decline halted. New hotels, restaurants and housing units have gone up, but the city's blighted neighborhoods have not seen as much rejuvenation. About 150,000 people moved out of the city over the past eight years, largely because of quality-of-life issues such as crime and education.

Where the candidates diverge most pointedly is on education policy: Katz favors school vouchers, Street does not. Katz also promises an aggressive tax reduction scheme, while Street promises more modest reductions.

Street touts his long experience as a selling point. Katz points to Street's record as part of a job left incomplete, saying Street has "glommed onto" Rendell's successes.

Katz, a long-standing finance consultant to numerous cities, touts himself as a font of fresh ideas from the business world. Street counters that Katz has no governmental experience and is little more than a "hired gun." Both major newspapers here have endorsed Katz, while Street has received the endorsements of big labor.

The candidates also differ in personal style. Katz has an easygoing demeanor, while Street seems perpetually scowling and has a reputation for playing hardball.

Because of Street's prickly style, some key black Democrats have defected from their nominee and thrown their support to Katz, including some black church leaders and John White Jr., a prominent political figure who lost to Street in the primary. In a highly unusual political move, Katz spent money during the Democratic primary last May on negative ads against White. Now, White supports him.

Katz and White go back to a time when they both worked on the first congressional campaign of Democrat William H. Gray III, which Katz ran. Katz's connections in the black community also go back to the 1983 campaign of Democrat W. Wilson Goode, Philadelphia's first black mayor.

The Committee of 70, a nonpartisan good government group, likens the current campaign to the one in 1987, which pitted Goode against the legendary former mayor Frank L. Rizzo, a white Democrat turned Republican.

In that race, most white Democrats crossed party lines and voted for Rizzo. The Committee of 70 predicts that same kind of defection will happen on Tuesday. When they have a choice, blacks tend to vote black and whites tend to vote white, the committee says in an analysis of the effects of party and race. Historically, though, blacks have shown far less willingness to cross party lines than whites.

If Philadelphia's white Democrats defect, the move would run counter to the growing number of big white-majority cities that have elected black mayors, such as Wellington Webb in Denver, Ron Kirk in Dallas, Lee P. Brown in Houston and Willie Brown in San Francisco. Philadelphia is 47 percent white and 42 percent black.

And blacks have voted for white candidates in some black-majority cities as well. This was most recently the case in Baltimore and in Oakland, Calif., where Democrat Jerry Brown is a white mayor in the majority-black city.

Here in Philadelphia, Rendell, a white Democrat, has enjoyed strong crossracial support.

"Indeed, crossracial voting shouldn't be so surprising anymore" in American politics, Fred Siegel, a senior fellow at the Progressive Policy Institute, wrote in Nov. 1 issue of the New Republic.

But although both candidates in the Philadelphia mayoral election are aggressively courting voters across racial lines, Philadelphia may not yet have caught up with the national trend.

CAPTION: GOP nominee Sam Katz must attract crossover votes to win City Hall.

CAPTION: Appearing at an event Friday at LaSalle University, President Clinton campaigns for John