Nate Silver and the staff at FiveThirtyEight are doing groundbreaking work, but, as they have noted, they have to base their forecasts on a scarce supply of public polls. In some cases more than half of these polls come from GOP polling outfits. This was one reason why FiveThirtyEight forecasts in North Dakota and Montana were so far off in 2012. In fact, in August of 2012 Silver forecasted a 61% likelihood that Republicans would pick up enough seats to claim the majority. Three months later Democrats went on to win 55 seats.
In 2012, Democratic senate candidates won in nearly half of the states where Mitt Romney beat President Obama (five of 12: North Dakota, Montana, Missouri, Indiana, & West Virginia), proving that senate races are not merely a referendum on the President or on any single issue but a choice between the two candidates on the ballot. Nate Silver predicted that Heidi Heitkamp had only an 8% chance of victory and Jon Tester had just a 34% chance. In 2010, he predicted that Majority Leader Reid had just a 16% chance and Michael Bennet had only a 34% chance in Colorado. All four are senators today because they were superior candidates running superior campaign organizations who made their elections a choice between the two candidates on the ballot. Only three Democratic incumbent senators have lost reelection in the last ten years, and our incumbents are once again prepared and ready
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.
Even worse for Republicans, they have tried to tame the tea party by nominating a slew of candidates that are too ideological for a general electorate (e.g. Personhood, pay equity, eliminating min wage, privatizing Social Security and Medicare)