Republicans face a very difficult Senate map this year, and when you combine that with the fact that Donald Trump is their presidential nominee, it's clear that a Senate majority is very attainable for Democrats.

If you needed confirmation of that, here it is.

A key super PAC tasked with defending Republicans' Senate majority has reserved nearly $40 million in ad time in the fall. The Senate Leadership Fund announced the following ad buys in a press release Tuesday afternoon:

  • New Hampshire – $15.8 million (four weeks in September; two weeks in October) (this buy is in concert with a New Hampshire super PAC)
  • Ohio – $8.1 million (two weeks in September; two weeks in October)
  • Nevada – $6 million (three weeks in September; three weeks in October)
  • Pennsylvania – $6.2 million (three weeks in October)
  • Missouri – $2.5 million (three weeks in September)

The group also said in the release that it plans to reserve ad time in the coming weeks in a sixth state, Florida, where Sen. Marco Rubio (R-Fla.) just opted to seek reelection, and possibly other states.

These are merely ad reservations, which can be canceled at any time. But they are generally a good preview of where those most closely involved in the races see the real battlegrounds shaping up. And what's notable on this list is not so much the races that are included, but those that aren't. Specifically, the group isn't yet reserving any ad time for Sen. Mark Kirk in Illinois or Sen. Ron Johnson in Wisconsin.

Similarly, ad reservations placed by Senate Republicans' official campaign arm — the National Republican Senatorial Committee — in April focused on many of the same states: New Hampshire, Ohio, Nevada (the one offensive opportunity on the list) and Pennsylvania. The NRSC did reserve some ad time in Wisconsin, but the small $2 million reservation there was only about one-third the size of its others. It did not reserve time in Illinois.

At the time, both Senate Republicans and Senate Democrats focused their ad buys on swing states where it would be much more expensive to purchase time later in the campaign — when the presidential campaigns start buying up time. In that way, leaving off Illinois and shortchanging Wisconsin (which was contested in 2012 but has long been blue), could be seen simply as Republicans not needing to buy up time this early.

But it's also clear that these are two incumbents facing very tough races, and it's not clear that Republicans see them as worthwhile investments at this early juncture — especially with their majority increasingly in doubt by virtue of Trump's nomination. What's more, the fact that the Senate Leadership Fund is reserving time for Sen. Roy Blunt in Missouri — a second-tier Democratic target — rather than Wisconsin (which Trump might try to win) suggests an even more defensive posture.

Potentially conceding one or even two states, of course, would not be ideal for Republicans. They can afford to lose four seats — or five if they can pick off a Democratic seat in Nevada. But once Republicans lose a net of four seats, they would lose control of the Senate — assuming Hillary Clinton wins the presidential election at the same time. (For what it's worth, it's unlikely the GOP would lose four Senate seats if Trump were winning the presidency, so four is really the magic number here.)

Conceding half the seats Democrats would need might not seem like a great strategy, but it's possible they might simply be too hard to defend. Kirk, after all, is seeking reelection in a state that has voted double-digits for Democrats in every presidential election since 1988, and multiple polls already have shown Johnson trailing by double digits in his rematch with former senator Russ Feingold (D).

Again, Republicans could jump into these races whole-hog at any point, but it will get more expensive as things move forward, and the fact that they haven't yet is notable. Whether they do or don't in the future, their ad reservations will tell the tale.