Like a crime boss in a pinstriped suit who is now hailed as a pillar of the community, super PACS have gone legitimate. At the Democratic Convention in Charlotte, according to Politico, both Obama campaign manager Jim Messina and White House chief of staff Jack Lew made presentations to would-be super PAC donors. Republicans are even more unabashed. Just 100 hours after he was tapped as Mitt Romney’s running mate in August, Paul Ryan was at the Venetian Hotel in Las Vegas pledging allegiance to super PAC impresario Sheldon Adelson.

Small wonder that campaign reporters covering the presidential race have fallen into the habit of equating candidate spending with super PAC efforts on their behalf. Domenico Montanaro, writing about TV ad buys for the influential NBC News blog “First Read,” stated this week, “President Obama’s campaign is still the top single spender, but with all the outside groups factored in, Romney and his supporters are outspending the president $307 million to $276 million.” Similarly, a Tuesday Associated Press story by Jack Gillum uncritically accepted Messina’s argument that “although Obama barely out-raised Romney, Republican super PACs in swing states had been out-spending Obama’s campaign 2-to-1.”

The campaign finance beat presents a tricky challenge in a year when unregulated cash is floating around the political system like gold dust in an 1848 California mining camp near Sutter’s Mill. So it is easy to assume, as most campaign reporters do, that a $10 million super PAC contribution has the political potency of an equal amount given to the Romney or Obama campaigns. After all, a mendacious attack ad is a mendacious attack ad, no matter who pays for it.

This logic would be unassailable were it not for the almost completely forgotten 1971 campaign reform bill. That pre-Watergate legislation contains a provision that serves as the closest equivalent to kryptonite in diminishing the mind-bending powers of super PACs. During the final 60 days of the campaign, broadcasters and cable systems are legally obligated to offer all political candidates the “lowest unit charge” for their commercials.

That means that in, say, the hotly contested Cincinnati media market, the presidential campaigns (and county coroner candidates across the river in Kentucky) cannot be charged more for a 30-second spot than a favored corporate advertiser with a volume discount like McDonald’s. But if super PACs or other independent groups want to buy the same coveted Cincinnati TV time, it is subject to the whatever-the-market-will-bear pricing that enriches TV stations and is a hallmark of the unfettered capitalism that Republicans revere.

(The best primer that I have found on how these FCC laws and regulations work in practice was compiled by Washington broadcasting lawyer David D. Oxenford. It explains, for example, that stations are obligated to sell reasonable amounts of advertising time to candidates for federal office. As a result, stations cannot bump commercials by presidential campaigns because a super PAC is offering to pay double for the same ad time).

How much will the presidential campaigns save because of the lowest unit rate—and how much more ad time than super PACs can they buy as a result? These numbers are virtually impossible to calculate since it depends on ratings, the type of time slots being purchased, the economics of individual TV stations, the competitiveness of each media market, and the size of standard commercial discounts. Despite the lack of a simple rule of thumb, election and media lawyers suggest in interviews that these discounts can be as much as 50 percent.

The 60-day lowest-unit-rate rules for the presidential campaign kicked in on September 7, as the political world was racing for the Charlotte airport. You would think that this important milestone might have been noted in a campaign year when every nuance of political strategy is dissected daily. Wrong. About the only mentions that I could find in NEXIS and Google searches were a one-paragraph reference in a Monday Wall Street Journal story by Amy Schatz and Suzanne Vranica and an equally fleeting walk-on in a New York Post article by Claire Atkinson.

It is always risky to construct over-arching theories about political stories that do not get written. But, forgive me, because in this case I cannot resist.

Walter Shapiro just chronicled his ninth presidential campaign. He writes the “Character Sketch” political column for Yahoo News. Follow him on Twitter @WalterShapiroPD.