Ryan Sager makes the point that most of the Democratic popular vote margins came in safe areas, so there's really no mandate there -- just a disgruntled conservative base. Further, he writes that "Outside of changing course in Iraq, I think it’s fairly obvious that there really isn’t [any mandate] — especially when it comes to domestic policy."
I think that's precisely incorrect. Generally speaking, Republicans have been winning a lot of elections through national security and cultural appeals. They have been winning very few because people like their health care ideas. Indeed, if you look at Bush's tenure, you see a lot of radical judicial appointments and foreign policy adventures, but a clear and constant attempt to deceive the electorate into mistaking him for an economic progressive. Say what you will about the internal structure and implementation of Medicare Part D and No Child Left Behind, but they were constructed and sold to blunt progressive advantages on those issues.
Democrats have long had a more popular domestic agenda; they've just lost out on other issue areas. What happened in 2006 was that their traditional advantages on domestic policy (see the Rust Belt candidates) added to a newfound credibility -- or at least the right's loss of credibility -- on foreign policy. The mandate on national security is what they've needed. They've long had the more popular domestic agenda, which is why the right has been trying, in its corporatist and clumsy way, to co-opt it.
