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The campaign committee for Senate Democrats is none too pleased with an analysis by poll-watcher Nate Silver that says the Republicans are "slight favorites" to win at least the six seats necessary to take control of the Senate during the 2014 midterms.

Silver's prediction isn't unique in calling a Republican takeover a reality, it's his methodology that is. In response, Democratic Senatorial Campaign Committee (DSCC) executive Director Guy Cecil put out a memo noting the scarcity of polls on which Silver bases his work, and pointing out past races for which a lack of information has led to incorrect predictions.

On Sunday, Silver offered his prediction in a long article on his website, http://www.FiveThirtyEight.com, based on a variety of factors: the national environment, candidate quality, state partisanship, incumbency, and head-to-head polls. In an appearance on ABC's "This Week," he put the odds of a GOP takeover at about 60 percent.

"The Democrats' position has deteriorated somewhat since last summer, with President Obama's approval ratings down to 42 or 43 percent from an average of about 45 percent before. Furthermore, as compared with 2010 or 2012, the GOP has done a better job of recruiting credible candidates, with some exceptions," he writes.

The Democrat-held seats he said were most likely to change to Republican hands are West Virginia, South Dakota, Montana and Arkansas. Four other seats he rates as a toss-up: Louisiana, North Carolina, Alaska and Michigan. Silver also says that there is a "plausible" chance of a GOP pickup in Colorado, Iowa and New Hampshire.

Though Cecil conceded that Silver and his team were doing "groundbreaking work," he notes that more than half of available polls come from Republican shops. "In August, of 2012, Silver forecasted a 61 percent likelihood that Republicans would pick up enough seats to claim the majority. Three months later Democrats went on to win 55 seats," he wrote.

Cecil goes on to highlight two specific races where polls pointed in the wrong direction: Silver predicted that Sen. Heidi Heitkamp, D-N.D., had just an 8 percent chance of winning and Sen. Jon Tester, D-Mont., had just a 34 percent chance of getting reelected. In 2010, Silver also predicted that Senate Majority Leader Harry Reid, D-Nev., only had a 16 percent chance of beating tea party favorite Sharron Angle. Heitkamp, Tester and Reid are all in office.

"We don't minimize the challenges ahead. Rather, we view the latest projection as a reminder that we have a challenging map and important work still to do in order to preserve our majority," Cecil wrote in the memo. He also said that most Democratic candidates are beating their opponents on metrics like polling, fundraising and campaigning despite well-funded attacks from "special interests like the Koch brothers [and] the Tea Party."

"It's clear that Republican Senate candidates, even candidates favored by Washington insiders, are pandering to the far right and embracing the reckless and irresponsible agenda of the Koch Brothers that will prove costly in a general election. Democrats have strong incumbents, great recruits in Michigan, Iowa, West Virginia and Montana, and are playing offense in Kentucky, Georgia, and Mississippi," he said.

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Democrats chide Nate Silver for predicting a GOP Senate takeover