Philadelphia's Democratic mayoral primary presents a civic conundrum wrapped in a question. The winner of this six-way contest, conducted in the distracting light of late spring, will have the approval of perhaps one-fifteenth of the city's population. And in an overwhelmingly Democratic city, that could well make him or her the next mayor - or, given a possible independent candidacy and a perfectly reasonable Republican, not.

Fortunately for the voters trying to solve this puzzle, the candidates' resumés and political bases effectively make this a two-man race, though not one that's easily called. There is plenty to like among the other four Democrats - beginning with Nelson Diaz, a seasoned and thoughtful attorney, and ending somewhere short of Milton Street, who has a federal record and may live in New Jersey. But this battle is almost certainly between Anthony Hardy Williams and James F. Kenney, a pair of veteran legislators who have divided the city's political forces between them.

Kenney's 23 years on City Council and Williams' 26 in the state legislature make them the most experienced politicians in the field. Williams followed his well-known father and eponym, Hardy Williams, into the state House and Senate, where he is best known as an advocate of school choice, having bucked his party to facilitate charter schools and public assistance for private schools.

Before resigning from Council to run for mayor, as city law requires, Kenney showed independence, too. He successfully challenged the mayor and police commissioner to decriminalize marijuana possession, heading off thousands of needless arrests annually; almost lost his seat for opposing the abused and costly city retirement perk known as DROP; and parted with his relatively conservative South Philadelphia base and beginnings to support same-sex partner benefits.

Serious reservations

But more than their considerable experience, it's the conspicuous support of an array of wealthy interests that makes Kenney and Williams the top contenders - and makes for serious reservations about both.

The more than 40 labor organizations backing Kenney include John J. Dougherty's powerful electricians' union - which already wields significant sway over Council to the detriment of the city - as well as the public employee unions he would face at the bargaining table.

Williams' avid supporters, meanwhile, include the malcontent carpenters' union - which is still waging a rearguard war on the Convention Center's newfound viability - and the founders of the suburban securities trading and investment firm Susquehanna International Group. In 2010, the same trio of Main Line benefactors shattered Pennsylvania campaign contribution records (which is no mean feat) with seven-figure donations to Williams' failed gubernatorial campaign.

Deepening these ties, the senator has shared a top employee with a political action committee and an education nonprofit funded by the same supporters. Education tax credits Williams championed effectively subsidized the Susquehanna partners' donations to that nonprofit and others.

Kenney, too, has helped his backers, especially by forcing the city's ailing pension fund to disgorge millions in additional public employee benefits even though it's not halfway prepared to meet its obligations.

Evenly matched

Does the field offer an alternative to these two? Diaz is the most likely, with an up-from-Harlem personal story, impressive resumé, and considered policy proposals. But his extensive public service - as a Common Pleas Court judge, U.S. Department of Housing and Urban Development general counsel, and city solicitor - falls short of the political experience of the front-runners. Perhaps partly as a result, he has struggled to draw broad support.

Candidate Lynne Abraham enjoys considerable notoriety thanks to two decades as the city's staunchly law-and-order district attorney. But in positioning herself as a moderate reformer, she has frequently been at odds with a record that was uncompromisingly tough on violent crime and disturbingly mushy on political corruption and police misconduct.

On the other end of the longevity spectrum, former Philadelphia Gas Works executive and mayoral aide Doug Oliver offers much promise but too little heft to run one of the nation's biggest cities.

The candidates' platforms differ mainly in emphasis and nuance. Kenney's education priorities include universal high-quality prekindergarten and school-based social services; Williams, who once ran a charter school with mixed results, advocates stronger charter oversight and mayoral control of the district. Kenney would boost police resources and prisoner reintegration, while Williams stresses police oversight and alternatives to incarceration. Kenney emphasizes services for new immigrants, one of his causes on Council, while Williams proposes more incentives to buoy minority-owned businesses.

For two men representing different facets of the city - the passionate Irish Catholic son of a firefighter from East Passyunk; the even-keeled son of an African American politician from Cobbs Creek - they are a remarkably close match. But the balance of power in City Hall isn't so close. Because the unions backing Kenney already wield too much influence, The Inquirer's choice for the Democratic nomination is ANTHONY WILLIAMS.