Our new House ad tracker: Major Democratic groups have reserved $11 million more for TV than the GOP

Thanks to a Democratic source who tracks media buys, we've put together a comprehensive new chart of all ad reservations for the four biggest groups that work on House races: the DCCC and House Majority PAC for Democrats, and the NRCC and Congressional Leadership Fund for Republicans. (HMP and CLF both have close ties to their respective party leadership and can be thought of as the "official unofficial" House super PACs for each side.)

Until now, we'd been relying on media reports and press releases to keep track of these reservations, but our information has been incomplete and sometimes contradictory. That's because its availability depends on the skill of reporters and the forthcomingness of PACs and party organizations—qualities that are often in short supply. This has been especially true on the Republican side, which has generally been very reticent to share information about its ad buying choices, making it difficult to know exactly what's going on. (Democrats, at least, have been much more transparent.) Much better data is available, however, if you have access to it, and now, fortunately, we do.

And it’s important information. Despite the fragmentation of the modern media landscape, television and radio advertising remains a crucial part of political campaigns. This is particularly so for House races, where most candidates—even including many incumbents—have limited name recognition and need to get their message out to large groups of voters at once. Online ads are, of course, a growing part of the voter-contact arsenal, and mail remains a stalwart. But broadcast media, thanks to its expense, still tends to dominate most competitive races in terms of its share of overall spending.

And given their cost, TV and radio ad reservations in particular are a good indicator of where both political parties think their top opportunities are: If you're spending millions to support a candidate, odds are you're pretty sure she or he is someone worth backing—and that you've seen polling to prove it. Unfortunately, though, it can often be difficult to track down who's actually made reservations, and where, and for how much. They can also be a moving target, because an ad reservation, just like a dinner reservation, can be cancelled, changed, or re-booked for a later date.