by Theodoric Meyer, ProPublica.

Dark money groups that don’t have to disclose their donors spent hundreds of millions in this election cycle. And now we’ve got a better idea of the extent of their spending in one crucial swing-state advertising market.

As the final hours of the campaign ticked away, we challenged ProPublica readers to help us “free the files” in Las Vegas, which aired more political ads this election cycle than any other market in the country. The results indicate a leading conservative dark-money group and its affiliated Super PAC spent more than $9 million on television advertising in Las Vegas during the election — one in every five ad dollars spent in the market.

The two groups — Crossroads GPS, a “social-welfare nonprofit” started by Karl Rove in 2010, and American Crossroads, a Super PAC that is closely aligned with it — bought far more ads than other independent groups in Las Vegas. Together, they spent nearly as much as President Obama’s and Mitt Romney’s campaigns combined.

The statistics come from a ProPublica analysis of more than 800 ad contracts from the Las Vegas market obtained as part of our Free the Files project. While not comprehensive, the data includes most of the contracts for political ads broadcast on Las Vegas’ ABC, CBS, NBC and FOX affiliates in the last months of the campaign. The Vegas ad buys, which encompass spending on everything from the presidential campaigns down to House of Representatives races and even the Clark County District Court election, totaled more than $47 million.

While it was known that Crossroads GPS and American Crossroads were spending big in the election — they were expected to fork out at least $300 million in total — the Free the Files data on Las Vegas sheds light on the extent to which the two groups were able to dominate the air wars in key battleground advertising markets.

All told, six groups that do not disclose their donors — American Action Network, American Future Fund, Americans for Prosperity, the Center for Individual Freedom, Crossroads GPS and the Republican Jewish Coalition — accounted for at least 21 percent of the spending we tracked in Las Vegas. All of them support conservative candidates. Crossroads GPS alone spent more than $6 million. (A liberal dark money group, Patriot Majority USA, also bought airtime, but many of the ad contracts failed to distinguish its ad buys from those of Patriot Majority PAC, a Super PAC that is affiliated with the group, making it hard to track the groups’ spending.)

Five Super PACs accounted for another 13 percent of the spending. They included Restore Our Future, the Romney-supporting Super PAC; Priorities USA Action, which supported Obama; House Majority PAC and Senate Majority PAC, which supported Democratic candidates for Congress; and American Crossroads.

Whether all the spending swayed the races in Nevada is an open question. Both liberal and conservative groups spent heavily, and the election results were mixed: Obama won Nevada with 52 percent of the vote, but Dean Heller, a Republican, came out on top in the Senate race. Joe Heck, a Republican, snagged one of the state’s two competitive House seats, but Steven Horsford, a Democrat, took the other.

The $47 million spent in Las Vegas represents a tiny fraction of the estimated $6 billion spent in races around the country this election cycle. But it’s a detailed look at the epicenter of ad buys this election. KSNV, the city’s NBC affiliate, broadcast more political ads than any other station in the country, according to Kantar Media; KTNV, the ABC affiliate, broadcasted the fifth most.

Several factors combined to make Vegas the country’s political-advertising capital, said Elizabeth Wilner, who works in Kantar Media’s Campaign Media Analysis Group. Nevada is one of an ever-smaller slate of battleground states, and Las Vegas is by far its biggest market.

Airtime is also relatively cheap in Vegas; campaigns can buy more ads for the same amount of money than they could in Denver or Miami.

But the Free the Files data also underscores the central role played by groups that didn’t exist in the last presidential election, including Crossroads GPS and American Crossroads. Lisa Howfield, KSNV’s general manager, said her station’s political advertising revenue was up 242 percent compared with 2008.