The last great thing this country did, the pinnacle of American Achievement, was to put a human on the moon. That happened shortly after Nixon took office, and coasting on the inertia of Kennedy's vow at the beginning of the decade. Since Nixon took office this country has lurched rightward. Since the Conservatives took control of the narrative, declaring that we are a fundamentally right wing conservative nation, the United States has been in decline. We have not even had the will to rebuild on the World Trade Center site. Reactionary conservatism has ground this nation to a halt.
Now I speak of progressive values rather then liberal ones. We were always the forward looking nation. If we didn't always live up to our ideals (see: slavery) we believed that we could sort out our problems eventurally.
Since the reactionary take over of the discourse, we are told that everything is perfect now, nothing should ever change. Things were better in the hazy past when White Males controlled the government and minorities knew their place. Both parties have embraced this, and having ideals, having a desire to make things better gets you labeled a 'Marxist' or a 'fascist' by people who haven't the foggiest idea of what either term means (see: Glenn Beck)
When the Republicans took control of congress in 1994, Democrats took that as a signal that the only way they could be elected is to "govern from the center" (i.e. take a rightward shift), and this has been the operating mentality of the party ever since.
There is a place for conservatism in governance, but they should never, ever be allowed to run things. The job of the conservative faction is to slow down progress to a manageable level (progressives do have a tendency to get carried away with enthusiasm (see: The French Revolution)), but never ever be allowed to have enough power, as they have since 1968, to control things. As we have seen, the country has ground to a halt, immobilized by inertia, allergic to new ideas, and grinding into stagnation.