In 2012, the GOP nominated Mitt Romney, an establishment politician if there ever was one. This year, they nominated Trump. Did the party just go nuts in four short years, and, if so, why?
I think it is mostly happenstance. The economy is in better shape now than it was in 2012, and there have been fewer manufactured crises over the debt limit and government shutdowns over the last four years. The rise of IS and gay marriage probably played a role in firing up the reactionary base. The biggest change, however, is in the GOP candidates themselves.
Romney was shrewd enough to appropriate the immigration issue for himself. He was unopposed in his lane; his two ultimate rivals were relatively undistinguished and had little claim to be “outsiders.” In 2016, however, the establishment lane was the one that was crowded, Trump had the advantages of outsider status, celebrity, and free media, and Trump grabbed the immigration issue at the beginning of the campaign.
All of this suggests that if the GOP doesn’t split, and Trump goes away quietly (neither event is a certainty), the GOP will return to its 2012 “normal” state if it loses the election; in other words, everyone will go back to pretending that the GOP electorate believes in limited government rather than white nationalist politics, and blind obstruction of the Clinton agenda will be the order of the day.