On Monday, The New York Times’s resident oracle, Nate Silver, weighed in on an issue that CJR has been following closely: the impact of outside spending groups in the battle for control over the Republican party.

After Karl Rove’s decision last week to create a super PAC to defend GOP establishment candidates against Tea Party primary challenges, Silver examined campaign fundraising in Republican primaries over the last two election cycles. He found that when establishment candidates squared off against more hardline conservative opponents, the mainstream candidates generally held vast fundraising advantages even in races they lost.

“An analysis of Republican Senate primaries in 2010 and 2012 suggests that money is usually the least pressing problem for the incumbents and other establishment-backed candidates whom Mr. Rove’s group might be inclined to support,” Silver wrote.

He argues that these numbers show that Rove’s new group, the Conservative Victory Project, could backfire because it might generate sympathy for the hardline candidates it targets without addressing the real causes of establishment candidates’ struggles.

But Silver’s analysis of primaries did not include an important factor: the role of outside spending. Instead, his breakdown of political cash considered only direct fundraising by campaigns, and not spending by super PACs, nonprofits or other outside groups that can spend unlimited amounts on elections. While the larger traditional war chests amassed by establishment candidates are significant, the omission of outside spending is odd in a piece that seeks to make data-driven inferences about the performance of an outside spending group.

Why does this omission matter?

One reason is because, as we reported last week, outside spending in Republican primaries has in fact been dominated by hardline groups such as the Club for Growth and FreedomWorks. In addition, we found that these two groups were far more successful in picking winners of GOP primaries than they were in backing victorious general election candidates. These results suggest that outside money may in fact be a significant factor that is influencing primary outcomes.

It is also important to consider how outside spending may tip the balance toward insurgent candidates. As Silver points out, insurgents are often vastly outgunned in traditional spending, including in races where they are victorious. But this finding also demonstrates that the key for these insurgents is not to match their opponents dollar for dollar, but rather to cross a threshold of financial viability and name recognition that gives them a chance at victory. Silver notes that insurgents could fundraise off of mainstream efforts to marginalize them, but outside spending could also provide insurgents this crucial boost.

We emailed Nate Silver to ask him why he did not include outside spending in his analysis, but we haven’t yet heard back. We will update if he responds.

UPDATE (5:20pm—February 12, 2012): A second look at Nate Silver’s piece suggests that he may not have been disregarding outside spending, but instead was under the mistaken impression that super PACs contribute directly to candidates. The law prohibits super PACs, nonprofits and other outside spending groups from making direct contributions, or coordinating strategies, with campaigns.

This apparent error was noted by Politico’s Kenneth Vogel in a tweet in response to our post:

Kenneth P. Vogel @kenvogel Hmm. Unclear if @fivethirtyeight is also under mistaken impression that super PACs can give to candidates http://fivethirtyeight.blogs.nytimes.com/2013/02/11/new-rove-group-could-backfire-on-g-o-p/ … @sashachavkin

Silver’s post includes language in two places that erroneously suggests that outside spending groups can make direct contributions to candidates. “But if a candidate adds to her coffers through large one-off contributions, such as money from a super PAC or from her own savings, it may not go that far if the candidate is otherwise having a difficult time persuading voters of her merits,” stated Silver’s post as of Tuesday afternoon. Later in the piece, he describes a hypothetical scenario in which “Mr. Rove’s group intervenes and contributes $1 million to the establishment candidate, bringing him to $4 million total.”

Such contributions would in fact be illegal. We contacted Silver yesterday via email and Twitter, but haven’t yet heard back.

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Sasha Chavkin covers political money and influence for CJR's United States Project, our politics and policy desk. He has written for ProPublica, the Center for Public Integrity, and The New York World. Follow him on Twitter @sashachavkin.