All of a sudden, the Democrats’ grip on the Senate is looking a whole lot more tenuous. According to Nate Silver’s latest number-crunching, there’s a 20% chance the Democrats will lose the Senate outright. Even if they hang on as expected, they’re likely to take heavy losses, Silver writes at his FiveThirtyEight blog (newly transferred to the New York Times). On average, the model has Democrats losing 6.5 seats, leaving them with 52 or 53 senators.
The big problem states for Democrats turn out to be the traditional swing states—Pennsylvania, Ohio, New Hampshire, and Missouri. Those states are all close, but Democrats have failed for months to gain any ground in them, and indeed have lately been falling behind. The math still isn’t great for Republicans—they'd need to win 28 of the 29 races they have any chance in to gain an outright majority—but a big wave of that kind is definitely plausible.