The NRSC is also booking early to exploit its cash advantage over its Democratic counterpart. The NRSC had $30.4 million cash on hand at the close of the last reporting period at the end of February, according to Federal Election Commission reports, compared to $19.9 million for the Democratic Senatorial Campaign Committee.

Kevin McLaughlin, the executive director of the NRSC, signaled that the investment would lean heavily on attack ads aiming to define Democratic challengers, whom he said in a statement to POLITICO "have led a charmed life" thus far.

“This historic NRSC investment will expose every [Democratic] candidates’ fealty to the socialism-loving extremists running their party and lay waste to the fictional, slick, focus-grouped alter egos created for them by ... dark money groups," McLaughlin said. "The stakes have never been higher, and the contrast has never been more stark. Buckle your chin straps, and put in your mouth guards. It’s going to be a wild ride."

The reservations from the committee's independent-expenditure unit largely overlap with those made in recent weeks by the top Republican and Democratic super PACs focused on Senate races, though with several additional states included.

The largest investment is in North Carolina, where the NRSC is spending $7.3 million starting July 5 in the race between GOP Sen. Thom Tillis and Democrat Cal Cunningham, which is expected to be the most expensive in the nation. Super PACs for each party have already reserved nearly $50 million combined in fall ads in the state.