Republican groups are pumping millions into states usually thought of as Democratic strongholds, seeking to tip competitive House races into their favor. The result? Three big Democratic states — California, New York and Illinois–are the top targets for outside spending in congressional races, according to the Sunlight Foundation's Follow the Unlimited Money.

The 2010 U.S. Census triggered redistricting across the country, creating once-a-decade opportunities in some districts. So while President Barack Obama and his Republican rival, Mitt Romney, are ignoring three of the nation's most populous states — neither candidate has placed an ad there at least since Aug. 2, according to Federal Communications Commission data compiled by Political Ad Sleuth — the tight House races have drawn the attention of super PACs, 501(c)4 non-profits, and other spending organizations as they accelerated their spending through September to drop more than $30.3 million, Federal Election Commission data downloaded from Follow the Unlimited Money showed.

The numbers captured from the FEC are far from complete. Some types of spending groups, such as 501(c)4 non-profits, don't have to disclose spending outside the 60-day electioneering window before an election. As a result, the numbers below are a snapshot into the spending, but do not capture all of the outside expenditures made by some groups before Sept. 7. 

California — Congressional Outside Money in September:  $13.9 million

Thanks in part to redistricting, California is home to nine competitive House races, according to the Cook Political Report, and in September saw $13.9 million in congressional outside spending — compared to the $500,000 spent before September in all of the House elections, including primaries. In the most heated races, GOP-aligned groups are jumping in to defend incumbents from Democratic challengers. 

Two conservative groups, the U.S. Chamber of Commerce and the National Republican Congressional Committee, account for nearly $7 million in California, more than half of the total for the month. Overall, groups backing Democrats — such as the American Federation of State, County and Municipal Employees and the Sierra Club — have been outspent by groups backing Republicans, $7.9 million to $5.9 million. 

The NRCC, which spent in six districts across California, put a disproportionate $1.2 million into the 52nd district to oppose Democrat Scott Peters in his race against three-term Rep. Brian Bilbray. The organization, which disclosed relatively small ad purchases and survey research, has accelerated its activity. NRCC started spending in the general election mid-August with  weekly expenditures below $1 million, but in September unleashed $23.6 million across the country. 

The Chamber of Commerce had been inactive in House races until last week, when it spent close to $3 million on eight separate California races on Sept. 28, and followed up by dumping more than $1 million into two Illinois races a day later.

Illinois — Congressional Outside Money in September: $8.9 million

The Chamber's ad buys are some of the largest expenditures in the Illinois House races, one to help an embattled Republican Rep. Robert Dold in a now-Democratic 10th district, and the other to oppose Democrat David Gill as he fights for the open seat in the 13th district. Donations from Republican-friendly groups are outpacing their Democratic counterparts more than 2-to-1, while in the 10th, no Democratic groups weighed in last month. 

Illinois's 17th district received the most outside money. Freshman Republican Rep. Bobby Schilling is in a tight race with Democratic challenger Cheri Bustos. The NRCC again is a heavy-hitter in the race, spending almost $1.1 million in September, though House Majority PAC and the Democratic Congressional Campaign Committee pushed back with a combined $1 million.  

New York — Congressional Outside Money in September: $7.6 million

Right-wing spending groups spent heavily to boost Republican candidate Randy Altschuler in his rematch against Democratic incumbent Tim Bishop in New York's 1st Congressional District. According to Follow the Unlimited Money, Bishop has received comparatively little outside support in the general election, while Prosperity First, Crossroads GPS, and NRCC put more than $1 million into the race. Only House Majority PAC has come to Bishop's aid, with less than $130,000

In the 24th district, freshman Republican Rep. Anne Marie Buerkle has received $840,000 in support from outside groups, while only the DCCC has helped former Rep. Dan Maffei with $450,000 in media buys. Buerkle sits in a district that now leans Democratic and is at risk of losing her seat in November.