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Part of the reason for heavy spending in GOP primaries has been Republican battles over the party’s direction post-George W. Bush, as tea party and other conservative groups are increasingly challenging longtime incumbents, said Club for Growth President Chris Chocola, a former Republican congressman from Indiana.

“Some incumbents haven’t had primary challenges for years and years so they’ve been able to stockpile,” he said, adding that super PACs are a previously unavailable equalizer.

In the House, the dominant super PAC has been a Democratic group, the House Majority PAC, which has spent $9 million in anti-Republican ads. The pro-Democratic group has had more success attracting money from labor unions than Republicans such as the Young Guns Action Fund seemed to have soliciting money from business executives.

Sean Lengell contributed to this report.