But for ads by outside spending groups, the situation is very different. Stations can charge as much as the market will bear; they can be sued for defamation over the ads; and, perhaps most important, they are entitled to reject ads or demand changes in their content. Guidelines on the FCC website direct stations in general terms to “act with reasonable care to ensure that advertisements aired on their stations are not false or misleading,” but the agency says that enforcement is primarily conducted by the Federal Trade Commission. In turn, the FTC says that it only regulates product ads, and the Federal Election Commission says there is no law regulating the accuracy of political claims by outside spending groups. So even as outside spending groups’ ads have proliferated, there is no agency minding the store. Given free speech considerations, some people are glad about that.

Broadcast executives in Denver maintain that unless ads contain blatant falsehoods, it is their policy—and even their responsibility—to air political spots by outside spending groups. Michael O’Brien, director of sales for EW Scripps Company, the owner of KMGH, Denver’s second most-watched station, said KMGH requires supporting documentation for claims made in ads by super PACs and nonprofits. In 2010, KUSA pulled an attack ad by a nonprofit that falsely accused the much-maligned Congressman Perlmutter of voting to require health insurers to cover Viagra for convicted sex offenders.

But not one Denver station could recall a single political ad it had rejected in the 2012 elections.

“We’re a business, and we’re also stewards of the public airwaves,” Cornetta said. “Political speech is protected speech. To the degree someone wants to send a message, we believe we have a responsibility to allow them do so.”

When challenges to ads do arise, Cornetta added, it is usually because lawyers representing the target of the ad send a cease-and-desist letter to the station. The station then forwards the complaint to the advertiser, who must either refute the factual challenge or revise the contents. KUSA will not pull a spot without offering an advertiser a chance to respond. “A lot of times it just goes away,” Cornetta said.

Challenges from the stations themselves, meanwhile, seem to be far and few between. By opting for skillful deceptions instead of black-and-white falsehoods, outside spending groups appear to have identified a loophole that neither broadcasters nor the fcc have shown any appetite for closing. Jeff Harris, KMGH’s news director, argues that “Ads are much more sophisticated now. They’ve gotten defensible.”

Free Press, the media watchdog organization, analyzed the political advertising and news coverage of Denver’s four leading broadcast stations in during August and September. Timothy Karr, senior director of strategy for Free Press, contends that the approach most local broadcasters take—waiting for challenges and then demanding proof of blatant falsehoods before pulling an ad—falls short. The stations, he says, need to reject deceptive ads before they hit the air.

EW Scripps’s O’Brien said that a “common tactic” among political advertisers is to make outrageously false claims in the expectation that they will gain wide attention before eventually being taken down. “It’s a shock factor,” he said.

If the corporate side of local television stations were to screen out these falsehoods before they aired, the public might be better served. And if they drew the line at deceptions, and not just at inaccuracies, they might raise the level of political debate a notch.

In the newsroom: Is factchecking the answer?
When the stick-figure ad attacking Perlmutter was released, broadcasters were legally required to accept it, because it belonged to a federal candidate, Joseph Coors.

But two Denver stations gave it additional treatment on the air. KUSA’s “Truth Test” found the ad’s allegation—that Perlmutter was responsible for scamming taxpayers—was “a misleading statement.” KCNC’s edgier “Reality Check” segment declared the ad “DECEPTIVE,” with a bright red stamp. “Bottom line,” announced kcnc’s reporter, Shaun Boyd, “there is no evidence Ed Perlmutter was corrupted by his ex-wife’s lobbying.”

Sasha Chavkin covers political money and influence for CJR's United States Project, our politics and policy desk. He has written for ProPublica, the Center for Public Integrity, and The New York World. Follow him on Twitter @sashachavkin.