LET'S TALK ABOUT what happens when the mayor's race gets nasty.

It's Philadelphia. So (a) it surely will; and (b) pretty soon.

In a month and a day, one of six (or if we're being honest, one of three) candidates wins the Democratic primary on May 19.

So let's-get-nasty stakes are high for top contenders Anthony Hardy Williams, Jim Kenney and Lynne Abraham.

There are issues tied to gender, age, connections and candidate records during years of public service.

Who does what to whom and how will decide the winner.

Why?

Well, news from countless forums and debates hits voters only if a candidate doesn't show - or shows, then passes out.

Position papers on better schools, more jobs and safer streets flutter past average folks long weary of promises of better schools, more jobs and safer streets.

That leaves TV, direct mail, social media, personal contact and perception.

And what drives perception in a town where the most active voters are 55 and older? If you're thinking TV, you're thinking clearly.

So cue the purveyors of perception.

And remember, because of caps on what candidates can raise - and because these candidates seem to have the fundraising abilities of the Society to Save the Sandfly - lots of TV money is from "independent" groups.

Williams' campaign so far is benefiting from nearly $2 million in TV spending by the American Cities Foundation run by suburban charter-school advocates.

Kenney's camp's approaching $1 million in on-air backing from Building a Better Pennsylvania (labor) and Forward Philadelphia (teachers).

Abraham, one assumes, will use what she can raise for ads suggesting the guys might be for sale or rent but she's her own woman; "nobody's mayor but yours."

She might even mention that those pro-charter guys also gave Williams $5 million when he ran for governor in 2010 and that Kenney's labor backing is tied to IBEW Local 98 boss John "Johnny Doc" Dougherty, who can be, well, controversial.

There's fodder here but it requires delicate handling.

If Williams and Kenney spar over outside money, that could turn into a tug-of-war over who's better for the city, "Doc" or pro-charter Bala Cynwyd rich guys.

That, in turn, could turn voters off or help Abraham.

But if she gets an edge, maybe we get an ad on her $380,000-plus take from the city's Deferred Retirement Option Plan, known as DROP, and/or her own drop during that first TV debate.

There's always, well, Williams is from the Legislature and you know what the Legislature's like, or Kenney's from City Council and you know what Council's like.

Both served so long in bodies so bad (Kenney 23 years; Williams 26), and TV spots on perks, pay raises and pensions always are effective.

Abraham was district attorney for 19 years; gotta be a case or two to mold into an ugly ad.

Who gains most from attacks? One insider says, "At this point, flip a coin three times."

This gets to larger questions.

Outside funding seems sleazy. But every big campaign is fed by special interests.

Millions in outside money seems unfair. But what can be done about it?

Committee of Seventy boss David Thornburgh notes that independent expenditures are legal so long as they're not coordinated with campaigns.

"As long as Citizens United stands, there's nothing we can do about it," Thornburgh says, referring to the 2010 Supreme Court ruling allowing corporations and unions to spend unlimited amounts to promote or pummel candidates.

He adds, however, that finance reporting online every 24 hours would help.

(The deadline to report campaign money is May 8.)

As is, we have fragmented donor disclosure, unnecessary reporting delays, caps on candidates but no caps on "independent" groups, therefore potentially giving people who don't live or work in the city the power to pick its mayor.

In short, we already have a race that's nasty.

Blog: ph.ly/BaerGrowls

Columns: ph.ly/JohnBaer