This basic scenario is fairly common, but here in Philadelphia it's more obvious than in some places. An African-American incumbent is running against a white opponent who was once a Democrat. The city is overwhelmingly Democratic in its registration (75-17%), but the race will be much closer than that. Blacks will overwhelmingly vote for the incumbent, Street, and whites will overwhelmingly vote for the challenger, Katz.
At first glance, the logical answer is that the white voters are racist. Black voters normally vote for Dems, so it's normal for them to vote for one aside from his race. White voters are more likely to vote for a Dem if s/he's white.
I'm sure stark naked racism plays somewhat of a role here. Philly's race relations - not just black/white, but the whole complex issue - are not and have never been particularly good. For a city of this size and with it's general degree of cosmopolitanism, the locals are pretty resistant to assimilating the latest round of immigrants - Vietnamese, Latino, etc...
But, the truth is it's more complicated than that. Both sides, to some degree, are engaging in identity politics. A good deal of the white voters are not going to pull the lever for Katz because they're racists - they'll do it because they think he'll be a better mayor. They may be right. Some of the black voters are going to vote for Street because they identified with his plight after Ashcroft's boys bugged his office (polls swung in his direction after that).
However, some voters are going to vote for one over the other because they think he'll be a better mayor for them. Black voters, rightly or wrongly, likely tend to perceive that a black mayor would understand and pay more attention to issues that the black community is more likely to face, while white voters perceive things similarly about a white mayor. Some of that is tinged with racism, but some is just a rational (if potentially incorrect) appraisal of the situation.
I'm pretty much a dead dog Democrat these days, but I nonetheless approached Katz with an open mind. I chose to vote for Street for two reasons - the first, and it probably will cost him the election, was the way Katz behaved when the bugging scandal broke. I think if he'd taken the exact opposite position that he did, and acted livid about the FBI messing around during an election, he would have gained votes. He could have stayed above the fray and been statesmanlike, and the story would have still had all the press attention that it did. Bad move.
And, secondly, the centerpiece of Katz's economic plan is a disaster. We have an onerous city wage tax here - which you pay whether you work in the city or not (and pay a reduced rate if you work in the city and live elsewhere). For residents it's 4.46%. The perennial issue is how to have a revenue neutral tax reform which sharply reduces or gets rid of the thing. For the usual reasons it never happens.
Katz proposed reducing it by about 1 percentage point, and then borrowing $750 million over ten years in order to pay for it, while decreasing city spending by 1% per year (not sure if that's actual reductions or over current projected growth). The fantasy is that this will pay for itself as new jobs flow into the city. So, debt service will be covered by the resulting boom caused by the magical tax cut fairies.
It could happen - whether fortuitously or as a direct result of the tax reduction. But, it's a huge gamble. If it doesn't work, it'll put the city in a pretty dire financial situation. Hello, Alabama!