House Republicans have reserved $18.2 million in post-Labor Day television airtime, spread across 27 competitive districts.

According to a breakdown of the buy obtained by Roll Call, the National Republican Congressional Committee’s independent expenditure arm plans to invest heavily in the Boston, Denver, Las Vegas and Sacramento, Calif., markets, with a combined $8.6 million buy. The committee’s IE reservation is split between Democratic- and Republican-held seats — an early signal that the committee plans to go on the offensive and is not content to simply defend the GOP’s 25-seat majority.

The buy, reserved this week, marks the NRCC’s first foray into the fall advertisement air war. The Congressional campaign committees have reserved more than $100 million in television time for an air war that is shaping up as among the earliest and most competitive in recent history.

With the presidential campaign looming large in key battleground states and super PACs promising eight-figure advertisement buys in some markets, the four committees began reserving time as early as April to save costs and ensure prime placement for their spots.

The NRCC was the last of the four Congressional committees to reserve television advertising time for the fall. Committees can, and often do, pull out of media markets and enter new ones as races change. But for now, the specs of the NRCC’s first IE reservation include:

• $3.08 million in the Sacramento market, which covers competitive races in Rep. John Garamendi’s (D) 3rd district, Rep. Dan Lungren’s (R) 7th district, Rep. Jerry McNerney’s (D) 9th district and Rep. Jeff Denham’s (R) 10th district.

• $2.23 million in the pricey Boston market, which covers endangered Rep. Charles Bass’ (R) 2nd district in New Hampshire, as well as a competitive GOP challenge of Rep. John Tierney (D) in Massachusetts’ 6th district.

• $1.64 million in the Denver market. This buy covers competitive contests in Rep. Scott Tipton’s (R) 3rd district and Rep. Mike Coffman’s (R) 6th district. It could also include Rep. Ed Perlmutter’s (D) 7th district, but that race is less competitive at this point.

• $1.62 million in the Las Vegas market. This buy targets a highly competitive race in Nevada’s 3rd district, where Rep. Joe Heck (R) faces state Assembly Speaker John Oceguera (D). It could also cover the newly added 4th district, although that open seat is considered a likely Democratic pickup.

• $1.3 million in the Pittsburgh market. This covers the state’s recently redrawn, GOP-leaning 12th district. Attorney Keith Rothfus (R) faces Rep. Mark Critz (D) here.

• $1.21 million in the Charlotte, N.C., market. Rep. Larry Kissell (D) faces an uphill battle here to keep his 8th district seat.

• $1.01 million in the Cleveland market. This covers the competitive 16th district race between two House Members: Republican Jim Renacci and Democrat Betty Sutton.

• $948,000 in the Salt Lake City market to boost Saratoga Springs Mayor Mia Love’s (R) bid against six-term Rep. Jim Matheson (D) in the redrawn, GOP-leaning 4th district.

• $856,000 in the Minneapolis market. This buy boosts freshman Rep. Chip Cravaack’s (R) re-election bid against a to-be-determined Democratic nominee.

• $738,000 in the Wausau, Wis., market earmarked for Rep. Sean Duffy’s (R) tough re-election bid in the 7th district. Some of the reservation in the Minneapolis market also covers Duffy’s district.

• $722,000 in the Quad Cities, Iowa, market. This buy covers Rep. Bobby Schilling’s (R) tough re-election race in western Illinois’ 17th district and targets Rep. Dave Loebsack’s (D) 2nd district in northwestern Iowa.

• $657,000 in the Green Bay, Wis., market for Rep. Reid Ribble’s (R) race against business consultant Jamie Wall (D).

• $642,000 in the St. Louis market to cover two competitive races in downstate Illinois. The 12th and 13th districts are open seats.

• $564,000 in the Raleigh, N.C., market to target Rep. Mike McIntyre (D) in the GOP-leaning 7th district. The NRCC also made a $293,000 reservation in the Wilmington, N.C., market intended for this race.

• $441,000 in the Lexington, Ky., market. Rep. Ben Chandler (D) faces a rematch with attorney Andy Barr (R) in the competitive 6th district.

• $296,000 in the Youngstown, Ohio, market. Rep. Bill Johnson (R) faces a rematch with former Rep. Charlie Wilson (D) in the GOP-leaning 6th district.