Despite a very good map at the start of the cycle, Senate Republicans have had to spend more than 40 percent of their money on defense in the final weeks of the 2012 campaign.

A review of spending by the Republican and Democratic senatorial committees' independent expenditure arms -- perhaps the best way to suss out which races are the most competitive -- reveals that Republicans so far have spent 59 percent of their funds on Democratic-held seats (offense) and 41 percent on Republican-held seats (defense).

For a year in which the map was supposed to be overwhelmingly in their favor, that's a surprisingly even balance. And emerging Democratic targets in Arizona and Indiana are assuring the committee will have to play plenty more defense in the final four weeks.

Republicans are now spending so-called "IE" money in four states where they are playing defense -- Arizona, Indiana, Maine and Nevada -- and three where they are playing offense -- Montana, North Dakota and Wisconsin. (Republicans are also launching their first IE ad in a fourth Democratic-held state, Virginia, this week.)

The GOP also is also playing defense in a fifth state where the party is prohibited from spending IE money by a pact agreed to by Sen. Scott Brown (R-Mass.) and challenger Elizabeth Warren (D). The National Republican Senatorial Committee has sent plenty of money to the state through other methods that don't violate the pact.

NRSC Chairman John Cornyn (R-Texas). (Reuters)

That's five states Republicans are clearly at risk of losing. And it's crimping their style.

Meantime, the GOP has yet to spend money on many of its top-targeted seats -- in some cases because it doesn't need to.

Only one state appears to be a near-certain flip right now, and that's Nebraska. Neither party has spent money on a state that is likely to flip from Democratic to Republican.

Democrats, meanwhile, have spent IE money in four Democratic-held states where the GOP hasn't -- Connecticut, Missouri, New Mexico and Ohio.

There are various reasons why Republicans haven't spent money in these states.

Both sides have pulled out of New Mexico, where Rep. Martin Heinrich (D-N.M.) is looking more and more like the favorite, and Republicans have said they aren't likely to spend money in Missouri in light of Rep. Todd Akin's (R) foibles. The NRSC also doesn't really need to spend money in Connecticut (where self-funder Linda McMahon is running) and Ohio (where outside groups have spent more than $20 million for GOP nominee Josh Mandel).

Because of this, the NRSC's investments might suggest more defense than the committee is actually playing. (Democrats, by comparison, have spent 73 percent of their funds on defense.) But it's still a pretty surprisingly even balance.

(Side note: While Republicans are technically playing defense in Maine, the fact that it's competitive enough for them to spend money is actually a good thing for them, given independent former governor Angus King's huge early lead.)

The totality suggests a map on which Democrats are defending seven states and conceding one, while Republicans are defending five states. 

In order for Republicans to take the majority, they would have to win eight of those 12 competitive states -- or seven and the presidency, in which case Paul Ryan would cast the tie-breaking vote as vice president.

That's tougher odds than they were probably hoping for, even as the majority remains gettable.