In their sixth and final broadcast debate, the major Democratic candidates for mayor of Philadelphia finally succeeded tonight in getting on one another's nerves.

During the hour-long encounter at the National Constitution Center, Chaka Fattah and Michael Nutter went at it bitterly over race, with Fattah stunning the crowd by accusing Nutter of having "to remind himself that he's an African American."

Fattah and Dwight Evans went at it over the wisdom of Fattah's proposal to lease the airport as a way of funding a massive antipoverty initiative, with Evans calling the idea "half-baked."

And Tom Knox and Bob Brady almost went at it quite literally.

What prompted the Knox-Brady explosion was a question to Knox about whether he ever expected anything in return for the $500,000 that he has donated to Democratic candidates over the years.

Knox, seated next to Brady, replied that his contributions were not part of the "pay-to-play" politics he has decried during his self-financed campaign, but were intended only "to elect good Democrats to office."

Brady, who is the chairman of the city Democratic Committee, would have none of it.

"If you're giving $500,000 in contributions, you're looking for access," Brady said.

"You're looking for good government," Knox replied, his voice rising. ". . . There you go. Whenever you get a buck, you expect to give them access. That's the kind of guy you are."

"You were deputy mayor making a dollar a year," Brady shot back, "and grossly overpaid."

Knox then talked about how Gov. Rendell, in whose mayoral administration he worked, last week described Knox as someone who wouldn't be afraid to "kick some butt" to bring about needed changes.

"I can kick butt," Knox told Brady, "and yours will probably be the first one."

"Can I move my chair?" responded Brady. "I'm scared now."

If the Brady-Knox flap was the most dramatic of the night, Fattah's continued sniping at Nutter - much of it over race and crime - was the most sustained.

It stemmed from yet another discussion of a topic that has been a staple in these debates: Nutter's proposals to allow police to stop, question and frisk individuals suspected of carrying illegal weapons.

The other four candidates have said they oppose the idea, with several, including Evans, saying it could result in racial profiling and harassment of black people.

In defending the tactic, Nutter said that his plan is "not about race, it's about criminals," and added: "As a person who's been black for 49 years, I think I know a little bit about racial profiling."

When Fattah got his next chance to speak, he pounced, saying: "I'm sorry the councilman has to remind himself he's an African American."

The crowd gasped, to which Fattah replied, "Well, he said it."

Later, when stop-and-frisk came up again, Fattah kept at it, suggesting that Nutter had acted improperly during the campaign in sometimes speaking of Philadelphia's murder statistics in racial terms. Nutter has talked often about how many of the murder victims are African American and has likened the carnage to a genocide.

"We all signed a pledge not to make racial appeals. . . ." said Fattah, who is counting on a heavy black vote if he is to come from behind and win on May 15.

Nutter broke in. "I've made no appeal to race, congressman, and you know that's not true."

Afterward, Nutter called Fattah's racial remarks "silly," adding: "I said what I said [about being black] to reemphasize the point that as an African American I've had my own experiences with civil rights."

Fattah rejected any suggestion that he was trying to portray Nutter as not worthy of black support, saying Nutter was the one who injected race.

"As candidates we have to be careful not to make racial appeals," Fattah said. "I felt it was getting too far along that way."

The most recent polls show Nutter and Knox in a virtual tie at the top of the five-man field, with Fattah, initially considered the favorite, running third, followed by Brady and Evans.

Sparks flew to a lesser degree over Fattah's airport lease plan, which is central to his campaign.

Evans, who had accused Fattah of using "fuzzy math" in projecting that the lease could produce at least $150 million a year for antipoverty programs, ramped up his rhetoric this time, calling the whole idea half-baked.

Fattah dismissed criticism of his plan from both Evans and Nutter, saying that "none of what has been said about the airport deal is correct."

Almost lost in the sometimes vituperative back-and-forth was what might have been the first mention in the campaign of the possibility of higher taxes. And it came from Fattah.

The congressman was talking about the importance of making sure all Philadelphia children have access to after-school programs - an initiative he would like to fund with that airport money.

Should that money not be available, he said, he would cut other programs. Then he added: "If necessary, I'll even raise taxes to do it."

The debate, moderated by Chris Matthews, host of Hardball on MSNBC, was sponsored by NBC10, the Constitution Center, WHYY, and the Philadelphia Tribune.

Contact senior writer Larry Eichel at 215-854-2415 or leichel@phillynews.com.