Supporters of Republican U.S. presidential candidate Donald Trump cheer at a campaign rally in Costa Mesa, Calif., last month. (Mike Blake/Reuters)

The Donald Trump rampage — still hard to believe after nearly a year — is a symptom of something deeper and more profound: the Republican Party’s slide into complete incoherence.

Rarely has a major party’s establishment been so out of touch with its voting base. Rarely have so many experienced politicians (Jeb Bush, Scott Walker, Marco Rubio, Rick Perry, et al.) been so thoroughly embarrassed, and so cruelly dispatched, by a political neophyte. Rarely have feelings been so raw that one leading Republican (John Boehner) would publicly describe another (Ted Cruz) as “Lucifer in the flesh.”

What does the GOP believe in? There was a time when anyone with a passing interest in politics could have answered that question. Today, who knows?

This ideological disintegration has been years in the making. I believe one fundamental cause is that after winning the allegiance of millions of “Reagan Democrats” — mostly white, blue-collar, and Southern or rural — the party stubbornly declined to take their economic interests into account.

Traditional Republican orthodoxy calls for small government, low taxation, tight money, deregulation, free trade and cost-saving reforms to entitlement programs. If I were independently wealthy, that might seem an agreeable set of policies. Ditto if I were one of the “small-business owners” to whom GOP candidates sing hymns of praise.

Republican presidential candidate Donald Trump told a crowd of his Indiana supporters on May 2 that Cruz would lose the state by "the biggest landslide in history." (Reuters)

But most working-class Republicans are, get ready for it, working-class. They are more Sam’s Club than country club. They don’t own the business, they earn wages or a salary; and trickle-down economics has not been kind to them. Their incomes have been stagnant for a good 20 years, they have seen manufacturing jobs move overseas and job security vanish, they have less in retirement savings and home equity than they had hoped, and they see their young-adult children struggling to get a start in life.

This segment includes military families that have borne the awful weight of more than a decade of war. Repeated deployments to Afghanistan and Iraq have caused tremendous strain; “wounded warriors” have returned bearing grievous physical and psychological scars.

What adjustments did the GOP establishment make for these voters? None. Most of the governors, senators and former somebodies who ran for the presidential nomination, and failed, offered nothing but flag-waving pep talks and demagoguery on social issues — along with promises to stick with trickle-down orthodoxy and intervene in trouble spots around the world. Only Rick Santorum and Mike Huckabee, who were dismissed as yesterday’s news, seemed to realize that working-class Republicans even existed.

Did Trump cunningly craft a message for these orphaned voters, or did he stumble across his populist appeal by way of beginner’s luck? At this point, it hardly matters. He offers policies, however far-fetched, that address their wants and needs. He rails against the free-trade pacts that he says robbed the nation of manufacturing jobs. He promises not to cut entitlements and often hints at boldly expanding them. He pledges an “America first” foreign policy that withdraws from entanglements and eschews interventions.

Trump also plays on these voters’ insecurities, resentments and fears. He makes Hispanic immigrants and Muslims his scapegoats. He goes beyond attacking President Obama’s policies to also impugn his identity — in effect, portraying the president as the incarnation of demographic change that many white Americans fear. And Trump delegitimizes establishment Republicans by painting them as cogs in a system that is rigged to favor the rich and powerful. (In this, he’s basically right.)

Faced with Trump’s challenge, GOP grandees have failed to react in any meaningful way. Trump’s closest challenger for the nomination is the least-liked Republican in the Senate, a man who believes the party’s problem is that its presidential candidates haven’t been orthodox enough.

In no way do I minimize the ugly side of Trump’s appeal — the naked chauvinism, the authoritarian streak, the cynical appeal to his supporters’ worst instincts. But it is wrong — and, for the Republican Party, suicidal — to ignore the fact that he is doing more than merely rousing the rabble. Trump is filling a vacuum left by years of inattention to voters who have been patronized and taken for granted. The fissures he exposed in the GOP will not go away.

1 of 45 Full Screen Autoplay Close Skip Ad × Trump captures the nation’s attention on the campaign trail View Photos The Republican candidate continues to dominate the presidential contest. Caption Businessman Donald Trump officially became the Republican nominee at the party’s convention in Cleveland. Donald Trump speaks at a campaign event at Trump Doral golf course in Miami. Carlo Allegri/Reuters Buy Photo Wait 1 second to continue.

The party now seems on the verge of anointing a presidential nominee who does not subscribe to many of the party’s core beliefs — yet who has absconded with much of the party’s base. Post-Trump, Republicans will have a choice: They can develop new policies or look for new voters.

Read more from Eugene Robinson’s archive, follow him on Twitter or subscribe to his updates on Facebook. You can also join him Tuesdays at 1 p.m. for a live Q&A.