When Philadelphia's next mayor delivers his victory speech Tuesday night, he should take a moment to praise a most important ally: his super PAC. Before long, it could become a regular part of any winning mayor's speech.

In this open-seat race in the nation's fifth-largest city, the disparity in spending between super PACs and the official campaigns has been considerable. Heading into the weekend, the race's three highest-spending groups on TV all were super PACs, according to a Democratic source tracking the buys. One outside group, funded by out-of-town charter-school advocates, had invested more on TV ads than the other campaigns combined.

And the candidate expected to win, former City Councilman Jim Kenney, has not one but two super PACs working on his behalf, each of which has spent close to a million dollars. In a race that serves as this liberal enclave's de-facto general election, he might never have even been competitive without the aid of the outside groups.

"Super PACs won this race," said Dan Fee, a Democratic strategist based in the city.

It might not be long before most mayoral races—and other local contests—proceed the same way. Super PACs have made a name for themselves in federal races, where multimillion-dollar behemoths such as American Crossroads and Senate Majority PAC have become almost as important as the campaigns and party committees themselves. They're expected to play an even larger role in next year's Republican presidential primaries, when many of them will have far more money at their disposal than the campaigns themselves.

But super PACs might be poised to have the most influence in the races farthest away from the national spotlight. The largesse of a few wealthy donors can struggle to make a dent in hyper-competitive Senate and presidential races, where even tens of millions of dollars are just drops of water in the ocean.

In smaller races, however, a few million or even thousands of dollars can swing the outcome. And the potential for that kind of influence could encourage their proliferation in races that usually don't see the kind of big-time spending of their federal brethren.

"I would argue it has an exponentially stronger impact just because you're dealing with such a small number of voters," said Joel Searby, a Republican strategist.

Searby would know: He helps local realtors' associations set up super PACs to become involved in local races. And they've become involved in races with far fewer voters than the 200,000 or so who will participate in the Philadelphia mayoral primary.

One of Searby's super PACs got involved in a city council race in Palestine, Texas, a small town between Dallas and Houston. The winning candidate received a total of 87 votes, Searby said; the same candidate who received the support of his group's super PAC.

"There will be continued interest in how interest groups can bring outside money to bear on elections," he said. "It's clearly in the bloodstream of America political media."

It's difficult to track how much outside money has been spent on local races because, unlike in Philadelphia, that money doesn't always have to be publicly disclosed. But the money that is disclosed shows a clear upward trend in how much is spent by outside groups. According to data gathered in 21 states by the National Institute on Money in State Politics, $175 million was spent by them in 2006—a number that ballooned to $245 million four years later.

That's a fraction of the total expected to be spent in future local races.

"Trying to draw empirical numbers for local outside spending is probably impossible," said Peter Quist, research director for the National Institute on Money in State Politics.

Barriers to super PACs' proliferation at the local level still exist. Many of the local strategists and activists—unlike national groups with teams of lawyers—lack the wherewithal to create the committees.

"The challenge right now is really more of a logistical one; there are only a few people who know how to execute at that level," Searby said.

And it's uncertain that with all the money spent on national races, there would be enough left over for smaller contests.

"I'm not sure how far it's going to trickle down," said Neil Oxman, a Philadelphia-based Democratic strategist. "It's not as if there's a trillions of dollars out there for this stuff; there's hundreds of millions."

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