In a nondescript office building just south of Washington, researchers watch hundreds of political ads captured daily from the nation's television airwaves.

The ads and details about them are shipped to campaigns, parties and interest groups, providing strategists timely snapshots of their opponents' and allies' on-air activities.

Campaign Media Analysis Group (search), a political ad tracking service, has the marketplace cornered, sending information each day to Republican and Democratic party committees, many House and Senate candidates and several national interest groups.

But at least one other company, Nielsen Media Research (search), is trying to make inroads.

Still, for eight years, CMAG has been the place political professionals have turned for day-to-day pictures of the political advertising landscape as they determine where to run ads, how much money to spend and what messages to project.

"If you have the money, they're indispensable," said Bill Benoit, who studies political ads at the University of Missouri-Columbia. "It's extremely important for campaigns — or political parties or third-party groups — who want to keep tabs on what the other side is doing."

And, with laws now prohibiting federal campaigns from coordinating strategy with independent groups, Benoit said, CMAG data also is invaluable for allies looking for guidance about where to help out their preferred candidates.

The company started in the early 1990s as the research arm of National Media, a Republican media powerhouse. In 1997, Evan Tracey left that firm to create CMAG, hiring ex-Capitol Hill staffers and one-time campaign aides from both sides of the aisle. Since then, the company has grown, becoming a division of commercial ad tracker TNS Media Intelligence last year.

Despite Tracey's GOP background, CMAG is nonpartisan with clients from all political persuasions. "We're in the business of politics, we're not political," Tracey said.

Each day, his analysts — numbering roughly a dozen in an election year — monitor the nation's top 100 local media markets, which cover 85 percent of the country, as well as national cable networks and Spanish-language channels. By the fall, they will screen 500 political ads a day.

Within a few hours of a new ad being broadcast, CMAG's software plucks it from the airwaves. For anywhere from $15,000 to $150,000 or more per race, clients receive the ad, a story board breaking it down frame by frame, and charts and graphs showing where the ad ran, how many times it was broadcast, the estimated spending and the sponsor.

In the presidential contest, strategists say the information is useful mostly for after-the-fact analysis. Their own large media teams typically can track ads and spending themselves.

However, strategists call CMAG priceless for local, state and issue advocacy campaigns, as well as House and Senate races, allowing campaigns to know precisely what voters are watching and what their opponents and interest groups are saying.

"What they've done is perfected the ability to take public data, turn it around and make it accessible for you to use quickly to determine your strategy," said Kim Alfano Doyle, a Republican media consultant.

Before CMAG, candidates and parties mainly relied on networks of volunteers to tape ads aired in their districts using VCRs. Tapes would arrive at campaigns days later. That's now the exception rather than the rule.

With CMAG, strategists have same-day knowledge about what's going on in a race.

"It's very difficult to keep track of what's on the air and what's not on the air when you're working on 60 House races," said Carl Forti, a spokesman for the National Republican Congressional Committee. "This makes it easier on us."

Without CMAG, "we may loose a day in getting a response out to an attack," said Brad Woodhouse, a spokesman for the Democratic Senatorial Campaign Committee.

But there are drawbacks to CMAG. It does not monitor all media markets, and all of South Dakota is missed because markets there are relatively small.

That's where Nielsen hopes it can capitalize. The company covers all 210 media markets. It's service notifies clients of new political commercials in their race within two days of them airing. A weekly newsletter analyzing spending also is in the works.

Steve McMahon, a Democratic media consultant, predicted it will take others years to figure out how to compete with CMAG.

"They're political people who know what political professionals are looking for in the data," McMahon said.