Here's Sides: "Better candidates emerge when conditions in the country favor their party. As political scientists Gary Jacobson and Samuel Kernell have argued, strong candidates run when they have a better chance of winning. And in 2014 — as in most midterm election years — the playing field is tilted away from the president’s party. So we should see good Republican candidates emerging."
And, we have. There are now a dozen Democratic seats -- 14 if you include Oregon and Minnesota where if a specific candidate wins upcoming GOP primaries the race could become competitive -- that are genuinely contested, meaning there is a real candidate with a serious consulting team actively and successfully raising money. And there are only two Republican seats -- Kentucky and Georgia -- that appear to be in any danger at all.
According to Sides, the most likely outcome is Democrats controlling between 46 and 49 seats; "in nearly one-third of simulations, Democrats control 48 or 49 seats, suggesting that if future events break in their favor — for example, President Obama becomes more popular — their chances of controlling a narrow majority could improve," concludes Sides.
Electoral models are, of course, not the gospel truth. (One reason we like Sides and his work so much is that he grasps and acknowledges this.) But, the Monkey Cage model backs up the ratings changes we've seen from independent political handicappers and the reporting from the ground, all of which suggests Republicans are on the march.