Like snowflakes, no two Philadelphia mayoral races are quite alike.

But with four days to go, the 2007 Democratic primary is sharply distinct from any ever run in the city. Contribution limits, a millionaire's dough, and an ethics board kicking butt and taking names are among the factors that make this a year to remember. Here are some ways this mayor's race has been unique:

Scrounging for money

When City Council enacted campaign-contribution limits in 2003, many believed they'd be struck down by courts or wantonly evaded and ignored.

Neither happened, which meant candidates had to work hard at raising money in small clumps and plan their spending carefully. The result was fewer TV ads early on, and it meant an early frontrunner wouldn't get the usual big contributions from special interests anxious to curry favor with a perceived winner.

"I think Chaka Fattah [who led in early polls] would have been the clear winner by now without the campaign-finance laws," said Committee of Seventy President Zack Stalberg.

Bags o' cash

Philadelphia saw its first-ever self-funded millionaire candidate, Tom Knox, running in a race with others subject to contribution limits. That gave him mastery of the TV airwaves early, allowing him to vault into the lead by April.

His early spending prompted Council to create a "millionaire's amendment" to the campaign-finance law last year doubling the contribution limits.

A proposal to suspend the limits altogether was defeated when most candidates and civic groups opposed it. Knox by now has spent nearly $10 million of his own cash on the race, but was 10 points behind Michael Nutter in the most recent poll.

Stealth attacks

When Council enacted its campaign law, one gaping loophole was widely noted: It limited contributions to candidates, but nothing prevented a noncandidate committee from raising unlimited contributions and spending the cash to support or oppose candidates.

For most of the campaign that didn't happen, but a number of committees popped up in the last two weeks to run negative ads against Knox and Nutter.

Most of the financiers of those efforts won't be known until after the election and one, run by Washington, D.C., lawyer Donald Dinan, has simply refused to comply with state campaign laws.

Getting over race

The campaign has, until very recently, been strikingly devoid of even indirect racial appeals, and polls show strong patterns of racial-crossover support among both African-American and white voters.

"We'd like to think it's because we're maturing as an electorate," said Temple political science professor Joe McLaughlin. "It may also be because none of these five candidates are seen as threatening to any group. They're experienced politicians, but they don't engender the kind of intense reactions Frank Rizzo did when he ran, or that John Street did in the 1999 race."

Egads - substance?

Maybe it was the dearth of TV ads, but Penn scholar Phyllis Kaniss, author of "The Media and the Mayor's Race," said conversation and news coverage in this race have been more issues-oriented than in the past.

"And the forums were very interesting and well-attended," Kaniss added. "Day in and day out, citizens would have two or three chances to hear candidates speak directly. It was a tremendous outpouring of civic interest, and that's really encouraging."

Ethics cops

For the first time, someone is actively enforcing campaign-finance laws. The new city Ethics Board has forced Fattah to refund some campaign cash and has launched several investigations of candidates.

The newly empowered board was an outgrowth of a City Hall corruption scandal. Stalberg said it's heartening that "the honesty-slash-competence issue has really dominated the campaign more than anybody expected." *