WASHINGTON—House Republicans have reserved more than $18 million in ad time in districts from California to Massachusetts as they look to protect their majority.

The ad reservation, from the independent expenditure arm of the National Republican Congressional Committee, could potentially include 25 congressional districts. The reservation includes a mix of seats currently held by Democrats and seats where Republican incumbents are facing strong challenges.

As with House Democrats, who earlier this year announced about $32 million in ad reservations, the NRCC could shift its spending to different districts or increase its spending as races develop. But the ad reservation offers an early idea of where Republicans think they will play offense and defense this fall.

Based on this early reservation, Republicans see opportunities for pickups this fall in California, Pennsylvania, Utah and North Carolina -- all states where district boundaries were redrawn after once-a-decade congressional redistricting.

The reservation includes $3 million for ads in the Sacramento, Calif., area, where Republicans are targeting two Democratically held seats and defending two GOP seats. In Pennsylvania, Republicans are holding $1.3 million in air time in the Pittsburgh area, where they hope to unseat Democratic Rep. Mark Critz. In Utah, the GOP has set aside nearly $1 million to boost Mia Love, a GOP challenger running against Rep. Jim Matheson, a Democrat. In North Carolina, the NRCC has reserved almost $2 million for two Democratically held seats.

The reservation also includes money for ads in Democratically held districts in Kentucky, Massachusetts and Iowa.

Republicans are also looking to protect incumbents in a number of races.

In Nevada, GOP Rep. Joe Heck faces a strong challenge from state Assembly Speaker John Oceguera and will be backed by more than $1.6 million in NRCC advertising, which may also be used for a new open seat in the state. In Colorado, the NRCC has set aside $1.6 million in the Denver area to protect two GOP incumbents, although that money could also be used to target Democratic Rep. Ed Perlmutter. Republicans have reserved more than $2.1 million for GOP-held seats in Minnesota and Wisconsin, and have set aside as much as $722,000 to boost Rep. Bobby Schilling, R-Ill., and $296,000 for Rep. Bill Johnson, R-Ohio.

The ad reservation is also notable for areas where no time is reserved.

While Democrats have set aside millions for House races in Florida, Republicans as yet are not reserving time there. Democrats likewise set aside more than $3 million in the Philadelphia area but Republicans have only set aside time in the Pittsburgh area.

The Republican ad buy is notably smaller than the Democrats' reservation earlier this year, though more money could be pumped in later. Democrats need to net 25 House seats in November to wrest control from Republicans.

© Copyright 2012 Associated Press. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.