The Democratic mayoral primary has apparently passed another monetary milestone.

American Cities, the independent expenditure group formed to support State Sen. Anthony Hardy Williams, has purchased $940,000 in television ad time for the closing week of the campaign, according to two sources familiar with political ad placements on local stations.

That seems likely to constitute the largest concentrated ad buy for a Philadelphia mayoral candidate. It would exceed the spending rate of Tom Knox in 2007, when he paid $1.6 million for ads over the last two weeks of the campaign.

"That would be a record for a single candidate," said Elliot Curson, an advertising executive who has run political media campaigns in Philadelphia since as far back as the 1960s. "We've never seen anything like that. A half-million dollars a week, $600,000 a week, yeah, but not $900,000 a week."

Former Gov. Ed Rendell said he, too, could not recall anyone spending that much in the closing week of a local campaign.

"It is very, very large number," Rendell said. "It is going to dominate the media to an extraordinary extent."

Typically, Curson said, a candidate needs to spend about $400,000 a week for a television ad campaign to have a significant impact.

"You can really get people's attention with $400,000," said Curson, who early in the campaign produced a YouTube ad for Nelson A. Diaz. "Nine hundred thousand gives you double the saturation. For that kind of money, you are not going to be able to turn around and not see something about Tony Williams. I wonder if it is not overkill."

The buy brings American Cities' total media expenditures for the campaign, including television and radio, to about $5.2 million, according to the sources.

Joshua Morrow, a spokesperson for American Cities, did not return a call seeking comment.

The $5.2 million far outpaces spending by any of the candidates or other independent expenditure groups.

Williams' campaign, for instance, has spent $320,000, including about $67,000 in the last week on the first negative ad in the campaign.

The subject of that ad, James F. Kenney, has spent $564,000 on media.

Kenney is supported by two independent expenditure groups - Forward Philadelphia and Building a Better Philadelphia, which are funded largely by unions, including the American Federation of Teachers and the International Brotherhood of Electricians. Between them, they have spent about $1.37 million on ads for Kenney.

The American Cities ad buy this week represented a $420,000 boost over what had already been purchased for the campaign's closing days.

American Cities has been funded largely by three executives of the Susquehanna International Group of Bala Cynwyd. The three - Joel Greenberg, Arthur Dantchik, and Jeffrey Yass - are proponents of school choice. As such, they backed Williams' unsuccessful run for governor in 2010.

Through this week, their ad campaign has remained positive and focused on Williams' support of school choice.

Their latest ad, which started this week, stays true to form.

Rendell said he hoped American Cities would refrain from going negative through the campaign's finish. He took issue with an ad being run by Williams' campaign that reminds viewers of inflammatory statements Kenney made in 1997 in defending police use of force.

"The Williams ad is factual but unfair in my judgment," Rendell said. "It is taking a quote from 18 years ago. Every one of us in politics evolves. I believe Kenney has. This was obviously aimed at inflaming people and making him look racially insensitive, and clearly he is not. I'm not endorsing Kenney, but I just think that ad should not have been aired."

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