The National Republican Congressional Committee has reserved $18 million worth of ad space in 17 media markets spanning 25 competitive districts — one of the first windows into which districts the committee plans to pursue and defend this fall as they seek to retain control of the House.

The ad reservations are split about evenly between districts where the committee can play offense and where they must defend vulnerable GOP incumbents, reflecting the NRCC’s desire to go after Democratic seats even as it strives to keep its majority.

The markets involved are generally ones where there will be plenty of ad traffic in the presidential or other races, which puts a premium on reserving space early.

Below is a chart breaking down the NRCC reservations. (Note: The buys are made by media market rather than by congressional district, and in some cases, the money could wind up being spent on one of two or more districts.)

The committee is reserving time that could be used to defend some of Republicans’ most vulnerable members: Reps. Reid Ribble (R-Wis.), Sean Duffy (R-Wis.), Chip Cravaack (R-Minn.), Bill Johnson (R-Ohio), Charlie Bass (R-N.H.), Bobby Schilling (R-Ill.), Joe Heck (R-Nev.), Dan Lungren (R-Calif.) and Mike Coffman (R-Colo.).

But it also targets some of the most vulnerable Democrats including Reps. Jim Matheson (D-Utah), Mark Critz (D-Pa.), Mike McIntyre (D-N.C.), Larry Kissell (D-N.C.) and retiring Rep. Jerry Costello’s (D-Ill.) open swing district.

The NRCC has also reserved ad time in the Cleveland market, where Reps. Jim Renacci (R-Ohio) and Betty Sutton (D-Ohio) had their districts merged into a new seat with a slight GOP lean.

Though some of the media markets cover multiple districts, it remains to be seen whether the money spent in them will be dedicated to offense or defense for Republicans. While markets like Sacramento, Boston, Las Vegas, and Denver include some offensive opportunities, the GOP districts covered by them are generally rated more competitive than the Democratic districts — at least for now.

For example, it seems more likely the money in the Denver media market will be used to defend Coffman rather than to go after Rep. Ed Perlmutter (D-Colo.), who is a second-tier GOP target. Ditto Las Vegas, where Heck’s swing seat is seen as more vulnerable than the new Democratic-leaning 4th district.

But that doesn’t necessarily mean most of the money will be spent on defense since the current Republican reservations can easily be transferred into other districts. So for instance, if Lungren is looking safe, the GOP can spend money in Sacramento to go after Rep. Jerry McNerney (D) or Rep. John Garamendi (D).

In addition, the reservations are not binding, which means the committee can pull them at any moment if a race changes significantly.

The NRCC’s reservations overlap with what Democrats have reserved in several markets. Both committees have bought in Sacramento, Denver, St. Louis, Quad Cities, Minneapolis, Raleigh, Charlotte, Wilmington, Boston, Las Vegas, Cleveland, Youngstown, Pittsburgh, and Wausau.

To see where Democrats have reserved time, check out our recap from April and the second round from last week.