In the six presidential elections of 1968 through 1988, the GOP averaged 52.5% of the vote. In the six presidential elections of 1992 through 2012, the GOP crossed the 50% mark only once.
The grand Republican win of 2010 was the product of unusual circumstances: more than one third of all votes cast were cast by voters over 60, the oldest electorate in any election since 1982. That circumstance was unlikely to repeat itself in 2012, and it didn’t.
In 2012, the GOP ran on the most conservative platform since 1964. It lost the presidency by almost 5 million votes, just under 4% of the popular vote. It lost the Senate. It held a diminished majority in the House only grace to gerrymandering: Democratic House candidates won more total votes than Republican candidates.
Second, they were able to enact sensible conservative reforms to the excesses of the New Deal:
But here’s an ironic truth: the Republicans of the rejected moderate era succeeded much better at undoing the excesses of the New Deal and Great Society than the immoderate Republicans of today. Between 1969 and 1983, they repealed New Deal regulation of civil aviation, trucking, shipping and railways; New Deal regulation of consumer banking and finance; and a vast swathe of controls of energy production and pricing. They stopped the construction of public housing, replacing it with Section 8 vouchers. They closed Great Society programs like the Office of Economic Opportunity and Model Cities.
What have the immoderate Republicans of the Tea Party era accomplished? Bupkus.
What went wrong? Many things, but start with this: Tea Party Republicans terrified the country. In 2011, they came within inches of forcing an entirely unnecessary government default. In 2012, they campaigned on a platform of ending the Medicare guarantee for younger people (while preserving every nickel of it for the Republican-voting constituencies over age 55) in order to finance a big tax cut for the richest Americans. Through the whole period 2009-2012, senior Republicans engaged in strident rhetoric of a kind simply not used by major party figures since the demise of Burton K. Wheeler and Alben Barkley. “Death panels” and “Ground Zero mosques”; Michele Bachman, Herman Cain and Donald Trump taking turns as the Republican front-runner; speakers of state legislatures praying for the death of the president and a former speaker of the House denouncing the president as a Kenyan anti-colonial alien to the American experience—we could fill this page with examples of important Republicans currying favor with their voting base by behaving in ways that the non-base would regard as reckless, racist, or just plain repellent.