The "liberal hour" as defined by Mackenzie and Weisbrot was the time which spanned the elections of Kennedy and Johnson, and with them, an unprecedented faith in the American government's promises and possibilities. The faith in American progress and pro-activity that Kennedy had started was continued with Johnson. The "liberal hour" had promised everything to everyone. No better could this be seen than in Johnson's inaugural address, which hit the major themes of the Great Society. Health care, education, poverty reform, urban renewal, Civil Rights, as well as reaching across the nation's borders to expand the greatness of America abroad became the critical elements of Johnson's administration. Johnson once said, "“We’re the richest country in the world, the most powerful. We can do it all," in an attempt to articulate this hour to the world.
What ended up resulting which challenged and eventually timed out "the liberal hour" was this "overpromising" of liberal ideas. The overextending of American commitment in Vietnam ended up dooming the promises and possibilities of liberalism because people were unable to continually make the sacrifices in both costs and lives that the war was demanding. At some point, the nation ended up not believing what was being said from the White House, causing a major credibility crisis, killing off the liberal hopes of unified government. Another element which ended up hurting the liberalist cause was the urban unrest which emerged out of the mid 1960s. The cities, the largest recipient of government aid and programs, were not materializing according to the liberal hour's hopes. This caused another credibility gap between what government was promising and what it was delivering. When Nixon ran in the 1968 election on the basis of coalescing the "silent majority," a large plurality of this coalition consisted of people who saw liberal promises to the cities, literally, up in smoke. In the final analysis, the election of Nixon ended up doing much to bring the end of "the liberal hour."
There are a lot of things you could cite for this:
- The economic problems of the late 1970s and early 1980s. These led people to think that the liberal economic policies weren't working and it also led them to feel that the US was no longer rich enough to be generous on welfare and such.
- The fallout from Civil Rights Legislation -- when the Democrats lost the South, it made Republican domination more likely.
- A feeling of international weakness. This was partly due to the Iran Hostage Crisis of 1979.
- A general feeling that the US was losing its confidence as a world power and an economic giant. This happened largely during Carter's presidency and made liberalism look bad.
- The political skills of Ronald Reagan
All of these things can be seen as parts of the cause of the change you mention.