THE Donald Trump phenomenon is a great gift to pundits because it can be analyzed and criticized in so many different ways. But two shorthands seem particularly useful. First, Trump is essentially using the Republican primary to run a third-party campaign, not a right-wing insurgency. Second, Trump’s appeal is oddly like that of Franklin Roosevelt, in the sense that he’s a rich, well-connected figure — a rich New Yorker, at that — who’s campaigning as a traitor to his class.

These two elements of Trumpism are intimately connected. In American politics, the two-party system, no less than the Senate or the Supreme Court, has long served as a check on pure democracy, a means of elite control. So long as there are only two competitive parties, the political diversity of the country will be channeled through their sluice gates, and the (mostly upper-class, highly-educated, self-consciously globalist) people who run the parties will exercise disproportionate control over which ideas find representation.

This does not mean the two parties are interchangeable, a Republicrat conspiracy against the public. A clash between powerful elites can still be a very real clash, as recent Supreme Court decisions attest.

Nor does it mean that elites always get their way, even where there is bipartisan agreement. If they did, the Simpson-Bowles entitlement plan and comprehensive immigration reform would have passed many years ago.

But it does mean certain ideologies and worldviews get marginalized in national political debate. The libertarian who wants to cut defense spending, the anti-abortion voter who favors a bigger welfare state, the immigration skeptic who wants to keep Social Security exactly as it is … all these voters and many others choose the lesser of two evils every November, because neither party’s leadership has any interest in representing their entire worldview.

In certain ways this narrowing can be good for the republic. Elites can have wisdom that populists lack, certain ideas deserve suppression, and multiparty systems are more likely to hand power to extremists or buffoons. (It’s a good thing for the country that neither Henry Wallace’s effectively pro-Soviet leftism nor George Wallace’s segregationist populism outlived their respective third-party bids.)

And when the two-party system is functioning at its best, party leaders can integrate compelling third-party ideas, or even reorient a party entirely to react to a public discontented with its options.

But it has been more than four decades since the last such reorientation, and two decades since the last time a third-party candidate saw his ideas even co-opted by the major parties. Across the latter twenty years, the country has endured a series of disasters that had bipartisan fingerprints all over them. Yet the various movements that have arisen in reaction to those failures — the antiwar left, the Tea Party right, Occupy Wall Street – have yet to even unseat an incumbent president, let alone change the basic lines along which the two parties debate.

Which is where Trump comes in. So far he’s running against the Republican establishment in a more profound way than the Tea Party, challenging not just deviations from official conservative principle but the entire post-Reagan conservative matrix. He can wax right wing on immigration one moment and promise to tax hedge fund managers the next. He’ll attack political correctness and then pledge to protect entitlements. He can sound like Pat Buchanan on trade and Bernie Sanders on health care. He regularly attacks the entire Iraq misadventure, in its Bush-era and Obama-era manifestations alike, in a way that neither mainstream Republicans nor Hillary Clinton can plausibly manage.

And he’s coming at all these issues, crucially, from a vantage point of privilege — which his critics keep highlighting as though it discredits him, when in reality it lends his populism a deeper credibility. He’s the Acela Corridor billionaire (albeit tackier than most) who promises to reveal what the elites are really up to, the crony capitalist who can tell you just how corrupt D.C. really is, the financier who’ll tell you that high finance can afford higher taxes. It’s precisely because he isn’t a blue collar outsider that he may seem like a credible change agent: Because he knows Wall Street, and because he doesn’t need its money to campaign, it seems like he could actually fight his fellow elites and win.

He won’t, of course, but it matters a great deal how he loses. In a healthy two-party system, the G.O.P. would treat Trump’s strange success as evidence that the party’s basic orientation may need to change substantially, so that it looks less like a tool of moneyed interests and more like a vehicle for middle American discontent.

In an unhealthy system, the kind I suspect we inhabit, the Republicans will find a way to crush Trump without adapting to his message. In which case the pressure the Donald has tapped will continue to build — and when it bursts, the G.O.P. as we know it may go with it.