The high rollers hitting Las Vegas have been betting on politics this fall, with more political ads airing in Sin City than any other media market in the country.

The same trend holds true for Free the Files, where Las Vegas accounts for 2,418 political ad files — the most of any market in our database of political ad purchases at TV stations from around the country. Volunteers have helped “free” more than 790 of those files, logging up to $45 million in ad contracts in Las Vegas so far.

But we want a better picture of who’s behind the Vegas ad blitz, so we’re declaring an Election Day Challenge here at ProPublica. The mission? To liberate Las Vegas.

With your help, we want to deconstruct spending in the most-saturated political ad market in the country and to analyze the impact of outside spending in the presidential campaign and other races, including the hotly-contested race between Republican Dean Heller and Democrat Shelley Berkley for John Ensign’s old Senate seat.

You’ll notice an Election Day Challenge bannered around our site. To participate, log into Free the Files by clicking “free a file in Las Vegas.” You will be asked to confirm that the file is an ad contract (not an invoice or other document), and then answer four easy questions about each contract — political ad information that would otherwise be locked inside PDF files on the FCC's web site.

Many thanks go to the hundreds of volunteers who’ve reviewed political files already over the last six weeks. With your help, we’ve logged $550 million in ad contracts overall. The top 10 contributors on our all-time Free the Files leaderboard as of 12 a.m. Eastern, Nov. 7, as well as the top Election Day Challenge contributor (whoever reviews the most files between now and 12 a.m. EST, Nov. 7), will earn official Free the Files t-shirts.

As the pundits and pollsters descend, here are two things you can do for democracy this election day: go vote, and help us uncover outside spending by freeing a file!