Crossroads GPS is reserving more than $5 million for the effort to defeat Sen. Kay Hagan (D-N.C.) pictured above.(AP Photo/Gerry Broome, File)

The conservative super PAC American Crossroads and its nonprofit affiliate Crossroads GPS plan to reserve about $15 million worth of fall air time in five key Senate races, bringing its total investment for the post-Labor Day campaign to more than $20 million in six battleground contests.

The Crossroads organizations will reserve about $14.6 million in five states Democrats are defending: North Carolina, Louisiana, Arkansas, Montana and Iowa, the group told Post Politics Monday. The new reserves will come on top of about $5.6 million in fall broadcast and cable advertising time in Alaska American Crossroads reserved last month.

The spending blitz offers a glimpse at GOP thinking about the battle for the Senate four months from the Nov. 4 election. Republicans need to pick up six seats to win back the majority.

American Crossroads, which was co-founded by Republican strategist Karl Rove, will reserve more than $3.1 million in Iowa for the period between Sept. 9 and the election. Rep. Bruce Braley (D) and state Sen. Joni Ernst (R) are in a competitive race there.

The group will reserve about $1.7 million in Montana beginning Sept. 23, where Rep. Steve Daines (R) is running against Sen. John Walsh (D), who was appointed to replace now-Ambassador to China Max Baucus earlier this year.

Crossroads GPS is targeting a trio of Democratic senators running in Southern states that tilt Republican.

About $5.1 million will go to North Carolina, a state with several expensive media markets. There, Sen. Kay Hagan (D) faces state House Speaker Thom Tillis (R) in a contest that has already attracted heaps of outside cash. The reserve will cover the period between Sept. 30 and the election.

Crossroads GPS will reserve more than $2.5 million in Arkansas between Sept. 2 and the election, where Rep. Tom Cotton (R) is hoping to dislodge Sen. Mark Pryor (D); and about $2.1 million in Louisiana between Oct. 7 and the all-party primary election in which Democratic Sen. Mary Landrieu's main GOP opponent is Rep. Bill Cassidy.

Fall ad reserves are subject to change. The groups said the current wave of reserves are initial purchases, suggesting it could be planning more in the coming months.

Notably absent are three other Democrat-held seats currently in play: Colorado, Michigan and New Hampshire.

Crossroads GPS is in the midst of a nearly $2.3 million summer ad blitz in Colorado, where it is trying to help Rep. Cory Gardner (R) unseat Sen. Mark Udall (D). In New Hampshire, former senator Scott Brown (R-Mass.) remains an underdog against Sen. Jeanne Shaheen (D). Michigan polls show a close contest between Rep. Gary Peters (D) and former secretary of state Terri Lynn Land (R).

Taken together, the two organizations are currently in the midst of four-state, $9 million-plus summer spending spree.

The Crossroads groups say they also intend to play in House races. But to date, they have only gotten involved in one race: New York's 21st district contest. There, American Crossroads spent money in the GOP primary to help former Bush administration aide Elise Stefanik.

Sean Sullivan has covered national politics for The Washington Post since 2012.