(Bloomberg Opinion) -- For a long time it’s been a cliché of American political commentary that the hollowing out of the country’s center is mainly the Republicans’ fault. “Asymmetric polarization,” analysts call it: The right moved right and the left didn’t move that much. Until recently, this was largely true. It isn’t true anymore.

For the moment, at least, the energy in the Democratic Party is all on the left – and I mean hard left. Alexandria Ocasio-Cortez, the charismatic and camera-ready new congresswoman from New York, champions the Democrats’ current zeal for symmetry. The filmmaker Michael Moore, for one, calls her the leader of the party and wishes the Constitution could be changed so that she doesn’t need to wait until she’s 35 to be president (she's 29). Her team should speak to Trump and the Republicans about that: They might be open to an amendment.

Let’s not get ahead of ourselves. Ocasio-Cortez isn’t actually leading anything and she isn’t running for president. But you’d have to be blind to fail to see where she’s pushing the party. Bernie Sanders is no longer an outlier. He and other actual presidential candidates such as Elizabeth Warren and Kamala Harris have made hard-left policies their signature positions. They want higher taxes on the rich, not just to raise revenue but because wealth is bad. They favor big new transfers to those on low incomes. They want capitalist finance reined in aggressively. And their approach to health-care reform disdains Obamacare incrementalism, calling for a Great Leap Forward to a fully government-run system.

Now AOC has given us her program: The “Green New Deal.”  This document is interesting not just for its content, but also for the reaction it elicits from many Democrats. The  plan might not be entirely practical, they suggest, and her numbers (if she had some) probably wouldn’t add up, but as a statement of progressive aspirations for social justice and environmental responsibility, it’s a valuable contribution and worth discussing. To me at least, this treatment seems a bit patronizing: Nice work, Alexandria.

When sympathetic commentators go to the trouble of thinking about the numbers, the prevailing tone is indulgent. My colleague Noah Smith estimates that her program might cost upward of $7 trillion a year. That’s more than double the federal government’s existing revenues and about one-third of U.S. gross domestic product. Really quite a lot. Noah regrets he could not go along with such a plan and calls it “troubling.” I’d rather put it this way: “This is your plan? You’re joking, right?”

Ocasio-Cortez’s Green New Deal calls for root-and-branch social and economic transformation. Taken at face value, its environmental goals are unachievable on the timescale she proposes – the U.S. cannot get to “net-zero greenhouse gas emissions” in 10 years, especially if it intends to rely only on renewables.  (The European Commission has proposed net-zero as a goal for 2050, and even that is ambitious.) But the green components of the program are its least demanding. Ocasio-Cortez recognizes, indeed boasts, that her plan involves a sudden and radical restructuring of America’s economy and society, and aims to ensure that ordinary Americans don’t suffer as a result. Hence, new government guarantees of high-wage employment, health care, housing, higher education and economic security for all. That’s how you get to $7 trillion a year and up.

This may not be “socialism” in the sense of Soviet-style command and control. You could also say it has American roots: FDR’s “Second Bill of Rights” and all that. Nonetheless, embracing this as a program, rather than as lofty aspirations not meant to be taken seriously, would require a cultural revolution. It would move the U.S. from its liberal tradition of individual responsibility and limited government to a collectivism somewhat to the left of EU-style social democracy. And, to repeat, this war-footing “mobilization” of people and resources is not an unforeseen drawback of AOC’s plan; it’s the whole idea.

The explanatory blog-post that resided briefly on Ocasio-Cortez’s website before being taken down – I’m assuming for the moment it was real and not planted by a malicious MAGA hacker – went further than the resolution drafted for Congress. Early draft or not, it surely makes you wonder about the mind-set of those involved. The question of financing is blithely waved away: The plan is an “investment program, not an expenditure.” The main concession to feasibility was to allow that it might not be possible to entirely phase out cows and aircraft within 10 years. All that was missing was the call for one world government, imagine no possessions, and let’s introduce a universal basic income for all personkind.

I’m sure most Democrats – politicians and voters alike – would think that Ocasio-Cortez’s plan, taken as a whole, is unaffordable and politically unworkable. The reforms most would like to see are more cautious and pragmatic, hence more feasible and effective (though still bold by U.S. standards). The party has noticed, too, that moderates did well in competitive races in the midterms. Moderates could build on that success by using the banner of Green New Deal to advance a feasible plan for mitigating climate change. They could make “Medicare for all” a public-option plan that moves toward universal access and puts downward pressure on costs without instantly nationalizing a large part of the U.S. economy. Yet out of a mixture of timidity and condescension, many of those same moderate Democrats are refraining from criticism and letting Ocasio-Cortez and her followers hijack those causes.

If they let this go on, it will be a grave tactical error. Progressive parties need a few radicals with wild ideas – and a lot of moderates to keep them firmly in check. Everywhere, but especially in the U.S., left-leaning parties do best when they combine ambition with clear limiting principles. Voters who see the need for change also want to see restraint, something Bill Clinton and Barack Obama understood.

A platform that entertains Ocasio-Cortez’s ideas as an admissible program for government casts limiting principles aside. A hard-left Democratic party, or even one that can be plausibly painted that way, is exactly what Trump would love to confront in 2020. Anybody who wants to see him beaten should regard Ocasio-Cortez and her ideas as a menace.

To contact the author of this story: Clive Crook at ccrook5@bloomberg.net

To contact the editor responsible for this story: Max Berley at mberley@bloomberg.net

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

Clive Crook is a Bloomberg Opinion columnist and writes editorials on economics, finance and politics. He was chief Washington commentator for the Financial Times, a correspondent and editor for the Economist and a senior editor at the Atlantic.

©2019 Bloomberg L.P.