The other shoe has dropped.

American Cities, the independent expenditure group backing State Sen. Anthony Williams for mayor, has launched a television ad campaign said to be worth $560,000.

American Cities joins two other independent groups - both supporting one of Williams' five rivals, former City Councilman James F. Kenney - in what is expected to become an increasingly acerbic air war to determine who will be the Democratic nominee for mayor of Philadelphia.

The groups supporting Kenney - Building a Better Pennsylvania and Forward Philadelphia - have already been running ads on behalf of Kenney.

Williams' campaign launched its own ad last week.

American Cities is being financed in large part by the same trio of Main Line financial traders who backed Williams' unsuccessful run for governor in 2010 with $5 million.

Joel Greenberg, one of three trading partners, said in a recent interview the trio could ultimately spend nearly as much on Williams' mayoral campaign. A source familiar with the campaign who spoke on condition of anonymity said the initial buy was $560,000.

As a third-party entity, American Cities is not constrained by the city's campaign finance restrictions on how much a person or a political commmittee can donate.

As an independent expenditure group, American Cities has no limits on its spending or contributions it can accept, but it is barred from coordinating its efforts with those of the candidate.

Greenberg and his partners, Jeff Yass and Arthur Dantchik, have supported Williams because he shares their passion for school choice.

It is noteworthy, then, that American Cities' first ad never raises the issue of education. Rather, the 30-second spot chiefly reminds viewers that Williams is the son of the late State Sen. Hardy Williams, a leading African American politician who once ran for mayor himself.

If elected, the ad contends, Williams will help create "a city where hope thrives and poverty dies. A city his father could be proud of."

chepp@phillynews.com 215-854-2594