It’s hard to believe that the United States, having resisted the siren song of socialism during its entire 20th-century heyday (the only major democracy to do so), should suddenly succumb to its charms a generation after its intellectual demise. Indeed, the prospect of socialist Bernie Sanders, whatever his current momentum, winning the Democratic nomination remains far-fetched.

The Dems would be risking a November electoral disaster of historic dimensions. Yet there is no denying how far Sanders has pulled his party to the left — and how hard the establishment candidate, Hillary Clinton, has been racing to catch up.

The Republicans, on the other hand, are dealing with a full-scale riot. The temptation they face is trading in a century of conservatism for Trumpism.

The 2016 presidential race has turned into an epic contest between the ethno-nationalist populism of Donald Trump and traditional conservatism, though in two varieties: the scorched-earth fundamentalist version of Ted Cruz, and a reformist version represented by Marco Rubio (and several so-called establishment candidates) — and articulated most fully by non-candidate Paul Ryan and a cluster of highly productive thinkers and policy wonks dubbed “reformicons.”

Trump insists that he’s a conservative, but in his pronouncements and policies, conservatism seems more of a rental — a three-story penthouse rental with Central Park-view, to be sure — than an ideological home. Trump protests that Ronald Reagan, too, migrated from left to right. True, but Reagan’s transformation occurred in his 40s — not, as with Trump, in his 60s.

In radically different ways, Trump and Sanders are addressing the deep anxiety stemming from the secular stagnation in wages and living standards that has squeezed the middle and working classes for a generation. Sanders locates the villainy in a billionaire class that has rigged both the economic and political system. Trump blames foreigners, most prominently those cunning Mexicans, Chinese, Japanese and Saudis who’ve been taking merciless advantage of us, in concert with America’s own leaders who are, alternatively, stupid and incompetent or bought and corrupt.

Hence Trump’s most famous policy recommendations: anti-immigrant, including the forced deportation of 11 million people; anti-trade, with a 45 percent tariff on Chinese goods and a 35 percent tariff on U.S. manufacturing moved to Mexico; and anti-Muslim, most notoriously a complete ban on entry into the U.S. Temporarily only, we are assured, except that the ban applies “until our country’s representatives can figure out what is going on” — a standard so indeterminate as to be meaningless.

Trump has limited concern for the central tenet of American conservatism (and most especially of the tea party movement) — limited government. The most telling example is his wholehearted support for “eminent domain,” i.e. the forcible appropriation by government of private property. Trump called it “wonderful.”

Trump has not yet called Vladimir Putin wonderful but he has taken a shine to the swaggering mini-czar who seems to run his trains on time. When informed that Putin kills opponents and journalists, Trump’s initial reaction was, “Well, I think that our country does plenty of killing, also,” the kind of moronic what-about-the-Crusades moral equivalence that conservatives have railed against for decades. Although, to be fair, after some prompting, Trump did come out against the killing of journalists.

Cruz is often lumped with Trump in the “anti-establishment” camp. That suited Cruz tactically for a while, but it’s fairly meaningless, given that “establishment” can mean anything these days. And given the huge gulf between the political philosophies of the two men. Cruz is a genuine conservative — austere, indeed radical, so much so that he considers mainstream congressional conservatives apostates. And finds Trump not conservative at all, as he is now furiously, belatedly insisting.

My personal preference is for the third ideological alternative, the reform conservatism that locates the source of our problems not in heartless billionaires or crafty foreigners, but in our superannuated, increasingly sclerotic 20th-century welfare-state structures. Their desperate need for reform has been overshadowed by the new populism, but Speaker Ryan is determined to introduce a serious reform agenda in this year’s Congress — boring stuff like welfare reform, health care reform, tax reform and institutional congressional reforms such as the return to “regular order.”

Paired with a President like Rubio (or Chris Christie or Carly Fiorina, to go long-shot), such an agenda would give conservatism its best opportunity since Reagan to become the country’s governing philosophy.

Unless the GOP takes the populist leap. In which case, a conservative restoration will be a long time coming.

© 2016, The Washington Post Writers Group