Congress generally doesn't get very good approval ratings, as a body; however, these low scores usually don't translate into big changes in the biannual elections. The reason is that most voters, despite not approving of Congress in the aggregate, do approve of their Representives. So, when you see polls that ask, "Who would you like to see control Congress after the next election cycle: the Democrats or the Republicans?" it doesn't really matter that the Democrats have a healthy lead. But a poll like the new one from NBC and The Wall Street Journal is something else entirely:

For big change to happen in the make-up of Congress, general dissatisfaction has to turn into specific dissatisfaction with the one Representative you have some control over. It appears that's happened.