As you can see, at 50 percent job approval in the RCP Average, the Arkansas and West Virginia seats are in deep trouble for Democrats. The seats in Montana, Alaska, Louisiana and South Dakota are vulnerable. If Obama were at 50 percent nationally, they would have a decent shot at winning Georgia, and have an uphill climb in Kentucky. Minnesota is an afterthought if Obama is at 50 percent, while Democrats would have to run well behind the president in places like Iowa, New Hampshire and Colorado to lose. North Carolina could go either way.

This is how most journalists seem to see the races right now: A few contests that are largely unwinnable by Democrats, some where they are in trouble but can win, and a bunch of others where Republicans might be able to win under the correct circumstances. This is the conventional wisdom that solidified in the spring of last year. It was the correct analysis at the time, when the president was at 50 percent.

But over the course of the summer, his job approval numbers slid into the mid-40s. The conventional wisdom didn’t follow. Given that movement, we would expect to see races in Colorado, New Hampshire, Virginia and Iowa become competitive, while Democrats in races in Michigan and Minnesota would start looking shaky. Individual polling started to suggest this, although it was largely dismissed.

With the movement of the president’s job approval numbers into the low 40s, the Democrats’ Senate odds would deteriorate considerably.