The nonprofit group Americans for Tax Reform is flooding several House races with more than $14 million in independent expenditures this month. 

The 501(c)4 lobbying group run by prominent conservative Grover Norquist has spent $1 million in six Senate contests, but its primary focus has been on bolstering Republican candidates for the House. Recent activity by ATR has made it one of the top spenders in a number of tight races, and in some districts the huge purchases have dwarfed all previous outside spending, according to Sunlight's Follow the Unlimited Money tracker.

Founded in 1985, the "dark money" group, which is not required to discloser its donors, is known for its Taxpayer Protection Pledge, which asks lawmakers and candidates "to oppose any and all tax increases." While its primary aim is to lobby Congress, ATR received attention earlier this month when it spent nearly $2 million in one day on competitive House races. The organization's ad spending has continued to surge since then:

• As Democrat Sal Pace tries to win the seat of freshman Republican Rep. Scott Tipton in Colorado's 3rd Congressional District, ATR has inundated the race with $1.6 million, compared to just $800,000 from all other outside groups combined. Though the race is tight, it's rated 'Lean Republican' by Cook Political report.
• In Ohio's 6th Congressional District, ATR has placed $3.1 million in ads to oppose former Democratic Rep. Charlie Wilson, who lost his seat in 2010 to Republican Rep. Bill Johnson. With less than $7 million in total outside spending, the purchases give Johnson an edge in the race, which leans Republican, according to the Cook Political Report. 
• ATR has dropped $2.5 million in ads in the fight over Pennsylvania's 12th, both to oppose Democratic Rep. Mark Critz and to support Republican Keith Rothfus. The 'Toss Up' race is the highest ranked House race in terms of outside spending, with a total of nearly $10 million. 
• In Georgia, the group has placed $1.1 million in negative ads against Rep. John Barrow, the Democratic incumbent in the 12th district. It also bought $130,000 in ads supporting Lee Anderson, the Republican nominee. The race is considered a 'Toss Up'.
• ATR is the top outside spender in New York's 21st district, where it spent $1.1 million against Democratic nominee Bill Owens.
• And in California's 52nd district,  ATR has ordered $1.9 million in ads to attack Democrat Scott Peters in his fight against Rep. Brian Bilbray. According to Cook, Bilbray is the "likeliest California Republican to lose."