The Campaign Finance Institute says 21 House members face challengers who have raised at least $50,000, double the level of the 2002 and 2004 election cycles, The Christian Science Monitor reported Monday.
"Many of these are not just individual challenges to individual incumbents, they are challenges supported by organizations that are engaged in battles for the soul of their respective parties," says Michael Malbin, CFI executive director.
One group active in backing challengers on the GOP side is the anti-tax Club for Growth.
On the left, labor unions and Internet political activists known as "netroots" have been engaged in a long-term campaign targeting so-called Bush Democrats.
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