So, what does Perlstein mean by 'the American consensus.'
He never explicitly says but the sense that I get from the book, which also ties in with what I know about 20th century American history, was that after the Great Depression and with the coming of the New Deal there was a new agreement between the government and the people. The new part, of course, was that there would now be a social safety net, including, but not limited to, Social Security.
American electoral politics for about 30 years, then, featured the two parties jockeying over what should and what should not be included. It also featured near total dominance of national electoral politics by the Democrats. From 1932 to 1968 the Democrats won seven out of nine presidential elections (the only two losses being to the most middle of the road Republican in history) and dominated in both the House and the Senate, often with super-majorities.
During this time the Republican 'brain trust' strategized that the only way to win any offices was to offer 'tweaks' to Democratic policies. This was also the period during which the armed forces were desegregated by executive order, the Supreme Court overturned Plessy v. Ferguson with Brown v. Board of Education and the Freedom Rides' and first sit-ins took place.
For reasons having to do with racism and possibly other resentments (but definitely racism) the old 'states rights' b.s. began to be heard again. Some movers and shakers (guys with money) began to sound each other out and plot a 'new course' for the Republican party. These were guys who were not 'professional' Republicans; rather they were businessmen who always voted Republican and wanted to take the party to the right. Little by little they discovered that they were not alone in feeling resentment toward the federal government and the New Deal.
Perlstein begins his book by putting us in the room with some of these early planners, e.g. Clarence Manion, who resented 'big labor' far more than anything else. To win more converts they realized that they needed to exploit white resentment of the (actually very slow) growth of the Civil Rights Movement.
As the story unfolds we see a rather disparate group of actors, each with an agenda that, while different from many of the other actors, coalesced around the idea of 'Conservatism.' Clearly, then as now, Conservatism meant different things to different people, but they all agreed that step one was running more conservative candidates for national office. They saw no value in running candidates who they viewed as Democrats by another name. Barry Goldwater was the man in whom most of these movers saw themselves reflected.
The story is fairly convoluted; Goldwater insisted for three years that he had no interest in running for President. Once he finally came aboard there was much competition as to who would run the campaign. Goldwater was often not consulted or listened to regarding his platform. Several times during the campaign he gave speeches which contradicted earlier pronouncements, especially regarding 'southern issues.'
In the end Goldwater was on the wrong end of one of the most lopsided Presidential elections in U.S. history. Through it all the campaign, and Goldwater, never took a backward, or even a sideways, step. A new ideology had entered American politics (or possibly reentered) and despite the predictions of the pundits of the time (the Republican party would take decades to recover) the way was opened for Nixon to win in '68 (southern strategy) and for the rise of Reagan in 1980. And for the end of the 'American Consensus.' Will we ever get it back?