The Senate GOP’s campaign arm reserved approximately $25 million in airtime today in six targeted Senate races — an early and aggressive move for a party looking to regain the majority.

The broadcast, cable and radio buy comes several months before the campaign committees’ independent expenditure arms typically start to reserve air time for the post-Labor Day campaign advertisement blitz.

“This is our first buy, not our last. But this foundation does ensure we will be heard through various channels as the electorate decides who they are going to vote for,” a National Republican Senatorial Committee official told Roll Call.

To be clear, the NRSC’s buy is just a reservation, and operatives can change the amount and move funds to different states before the ads hit the airwaves this fall. But the GOP committee’s purchase gives an early indication of where and how it will spend to net the four Senate seats the party needs to control the chamber in 2013. Mike DuHaime, who ran the NRSC’s IE operation last cycle, is again charged with overseeing the IE and, therefore, the bulk of the committee’s budget.

Senate Republicans are buying airtime early to lock in competitive rates and placement. Campaign insiders expect an avalanche of advertisements this fall from the presidential campaigns, Congressional committees and the growing ranks of new super PACs that will make it harder to purchase prime airtime in competitive states.

“Everyone knows that cost and inventory are going to be an issue in many states, rather than sit around and worry about it, we proactively dealt with the situation,” the NRSC official said. “This is helpful to our campaigns and gives NRSC donors confidence that we are maximizing their investment.”

A breakdown of the NRSC’s airtime reservation:

• $5 million in Wisconsin for the open-seat race to succeed retiring Sen. Herb Kohl (D).
• $5 million in Missouri to defeat first-term Sen. Claire McCaskill (D), who faces an extremely tough re-election race.
• $3.5 million in Montana to defeat Sen. Jon Tester (D), another first-term Senator with a hard re-election ahead of him.
• $5.5 million in Virginia for the open-seat race, where former Sen. George Allen (R) and former Gov. Tim Kaine (D) are locked in a highly competitive contest.
• $3 million in Nevada, where appointed Sen. Dean Heller (R) is seeking a full term against the presumptive Democratic nominee Rep. Shelley Berkley.
• $3 million for the open-seat race in New Mexico, where former Rep. Heather Wilson (R) and Rep. Martin Heinrich (D) are on track to face each other in November.

Notably absent from the large buy are a couple of states with potentially competitive races, such as Ohio, North Dakota and Nebraska. But this is only the first of many advertisement purchases the committee’s IE arm will make over the course of the year, the source cautioned. What’s more, the presidential campaigns are not expected to compete in North Dakota and Nebraska, so there’s less of an incentive for the NRSC to buy airtime early.

Also not included in the buy is airtime in the contested Massachusetts Senate race. But earlier this year, Sen. Scott Brown (R-Mass.) and the likely Democratic nominee, Harvard professor Elizabeth Warren, signed a pledge to pay penalties if outside groups spend on their behalf in the Bay State Senate race. As a result, any money spent by outside groups, including the NRSC, would be counterproductive.

The NRSC reported $16.2 million in the bank at the end of the February. The committee’s counterpart, the Democratic Senatorial Campaign Committee, reported more than that sum, $19 million in cash on hand, at the same time.

Check out Roll Call’s race ratings for all of the 2012 Senate contests here.