Its totals differ from actual spending reported to the Federal Election Commission in several ways. CMAG’s numbers reflect expenditures on broadcast TV ads, but not on ads aired on local cable or radio. They also exclude robo-calls and mailers that some groups must report to election officials. In some cases, however, CMAG’s estimates include TV ads that social-welfare nonprofits do not have to report to the FEC because of their content or the time frame in which they ran.

After the Supreme Court’s Citizens United decision in January 2010, which paved the way for unlimited corporate and union spending on federal elections, many predicted that super PACs would become the biggest vehicle for outside spending. Hundreds of super PACs soon sprang up, some of which paired up with c4s.

But it’s the sidekicks, the c4s, that have proved more muscular. Rove’s super PAC, American Crossroads, has spent an estimated $6.6 million on broadcast TV ads mentioning a candidate for president, CMAG data shows. Crossroads GPS has spent more than six times as much.

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