It is the physics of Philadelphia politics: For every new sign of campaign strength, there is an equal and opposite attempt to weaken that candidate.

"I must be doing good in the polls, because the attention seems to be focused on me," former City Councilman James F. Kenney quipped Monday morning after three of his five opponents in the May 19 Democratic primary election for mayor took aim at him during a feisty candidate forum.

Two polls last week - one by Kenney's campaign, the other by Forward Philadelphia, a group supporting him - showed him opening up a lead in the race.

A question about John "Johnny Doc" Dougherty, leader of Local 98 of the International Brotherhood of Electrical Workers, set off Monday's sparks.

Dougherty's union is helping to fund an independent political action committee, Building a Better PA Fund, airing campaign commercials supporting Kenney's election.

Kenney has had a complicated relationship with Dougherty, growing up as family friends, getting involved in politics around the same time, and then breaking in an acrimonious split in the Democratic Party before reaching a rapprochement.

Kenney acknowledged Dougherty's political power but then rattled off a list of unions and other groups supporting him.

"He will not have any undue influence," he said of Dougherty. "I mean, I know how to say no."

State Sen. Anthony Hardy Williams then interjected himself after Kenney noted that Williams has support from unionized carpenters. He claimed the Local 98-funded PAC was the reason Kenney entered the race.

"Jimmy Kenney said he wasn't going to run because he didn't have the money," Williams said. "Then, literally three months into the process, he decides to run because John Dougherty says, 'We'll take care of it.' That's the bottom line, and that's the truth."

Former Common Pleas Judge Nelson A. Diaz piled on, saying Kenney had "totally given himself" to Local 98's interest.

"You've got a PAC that's controlled by Johnny Doc that's putting millions of dollars into his TV ads," Diaz said.

Kenney ignored Diaz, who is trailing in the race, and focused on Williams, his closest competitor in last week's polls.

Kenney said he did not "get into this race at the behest" of Dougherty - and "I certainly didn't get into this race to promote the agenda of three hedge-fund billionaires who want to privatize our education system."

That was a reference to the three founders of Susquehanna International Group, a Main Line stock trading firm that is funding an independent political action committee airing ads supporting Williams for mayor.

The trio, like Williams, support vouchers to use tax money to pay for private school tuition, along with expansions for charter schools.

Forward Philadelphia is funded in part by the American Federation of Teachers, a national union that opposes the educational policies of Williams and his Susquehanna supporters.

A Forward Philadelphia poll released last week showed Kenney with 33 percent support among likely voters, nine points ahead of Williams.

Former District Attorney Lynne M. Abraham, who stayed above the fray in Monday's forum, was in third place in that poll with 18 percent.

Abraham and Kenney's campaigns are both launching 30-second television ads Tuesday. Williams was the first to launch a campaign-funded broadcast TV ad, on March 24.

Abraham's commercial shows her standing amid hundreds of pictures of young people.

"There's over 200,000 students in Philadelphia. Jim Kenney and Tony Williams are fighting over public schools vs. charters. I think they are both wrong," Abraham says in the ad. "It's making sure they all get a good education. .... It's wrong Philadelphia gets less school funding than other parts of Pennsylvania. I'll work with Harrisburg to change that. But if they refuse, I'll take them to federal court."

Her campaign paid $156,000 for a week of ad buys, according to a consultant unaffiliated with any of the campaigns who is tracking the purchases.

Kenney's new ad begins with him talking about his father, a former city firefighter.

"You know that early morning smell of burnt wood meant Dad has made it home again," Kenney says as pictures of his father in uniform flash by on the screen. "He never took the easy way of protecting our city, and neither do I."

Kenney then lists his advocacy of LGBT rights, ethics laws, and decriminalizing pot possession. He goes on, "I want stop-and-frisk gone, universal pre-K, and jobs that can support a family."

His campaign is spending at least $300,000 weekly on the TV ads, spokeswoman Lauren Hitt said.

At Monday's forum, still another rival took aim at Kenney: former State Sen. T. Milton Street Sr., who like Diaz and former PGW executive Doug Oliver has been polling in the single digits.

Street said Kenney's successful drive to decriminalize small-amount marijuana possession "doesn't make any sense."

Street, brother of former Mayor John F. Street, seemed to take umbrage when Kenney said he sometimes calls the former mayor for advice.

"He never called me," Street huffed in reply.

brennac@phillynews.com 215-854-5973 @byChrisBrennan