Ever since Bernie Sanders released details of his single-payer health care proposal recently, critics right and center have been on the attack against his “revolutionary, unaffordable and unachievable” scheme. In fact, for those who truly want to achieve universal, affordable health care, Sanders’ path is the only realistic way forward.

“Be reasonable: demand the impossible.” So said revolutionary Ché Guevara. [NOTE: I’ve since been corrected; the origin of this slogan was not Ché, but a graffiti encountered during the 1968 Paris uprising. Check it out here.] It’s a lesson much of the Democratic Party establishment needs to relearn this election year.

For instance, Henry Aaron of the Brookings Institution. One of the country’s top experts on social insurance and health care financing and a smart political observer to boot, Aaron ran a piece in Newsweek recently that took apart presidential candidate Bernie Sanders’s health care reform plan as being “radical in a way that no legislation has ever been in the United States,” vague on details, and technically unfeasible. It’s “a health reform idea that was, is, and will remain a dream,” Aaron writes. “Single-payer health reform is a dream because, as the old joke goes, ‘you can’t get there from here.’”

Above all, critics argue that Berniecare would undermine the one genuine domestic-policy achievement of the Obama years: the Affordable Care Act, which made it through Congress, however messily and imperfectly, in 2010, and which the White House has been fighting to implement, over fanatical Republican opposition, ever since. How could a loyal Democrat—or even a sympathetic Independent like the senator from Vermont—want to do such a thing?

Sanders’ critics are forgetting a few things. First, Obamacare remains highly unpopular, notwithstanding the fact that it contains multiple very popular elements. If a Republican is elected president in November, then unless the Democrats make big gains in the Senate, it’s possible that the GOP next year could repeal the whole edifice (while retaining a few popular features, of course.) The ACA has not stopped the slide toward high-deductible insurance plans that’s squeezing working families. And, for reasons not entirely the Administration’s fault, it’s a clunky vehicle that urgently needs to be improved if it’s going to survive.

So why shouldn’t we consider moving toward a single-payer system, such as Sanders advocates? The main reason, according to Aaron, is that it’s not “incremental.” Throughout American history, groundbreaking legislation like the Social Security Act, Medicare, the Homestead Act, and the Interstate Highway Act really weren’t very radical. That’s why they got through.

None overturned entrenched and valued contractual and legislative arrangements. None reshuffled trillions—or in less inflated days, billions—of dollars devoted to the same general purpose as the new legislation. All either extended services previously available to only a few, or created wholly new arrangements.

True so far as it goes—although “creating wholly new arrangements,” as Social Security certainly did, could be described as radical. So let’s take a closer look at Social Security, the crown jewel of the New Deal and the cornerstone of income security for millions of retirees, survivors, and disabled persons today. The program that Congress created in 1935 was extremely narrow in scope, ungenerous at times to the point of invisibility, and with a financing mechanism quite different from the one it has today. As Aaron notes, it took decades for the program to evolve into its present shape—or even to make much of a difference to most working people.

Just like Obamacare, you say. Surely that will change. So why do we need to be listening to Sanders talk about single payer?

Because something’s different this time. After the program became law, Democratic lawmakers got busy almost immediately putting together improvements. Less than four years later, the 1939 Amendments to the Social Security Act corrected a number of deficiencies. Steady work over the next decade, despite repeated attacks from the right, resulted in the 1950 Amendments, which again carried the program forward. And so on.

Obamacare became law in 2010. Since then, however, there’s been little effort to correct its faults. In part due to the constant attacks by Republican lawmakers and other powerful figures on the right, but also to the persistent timidity of the center-right Democratic establishment, hardly anything has been done to really improve Obamacare in the past six years. The recent delay in implementation of the “Cadillac tax” on health coverage was a nice handout to working families, but not even a permanent change, and it leaves open the question how the lost revenue will be made up.

Candidate Hillary Clinton has offered a few ideas for improving ACA: lower co-pays and deductibles, reduce the cost of prescription drugs, moving toward a system that rewards quality and value of care. Allowing Medicare to negotiate lower drug costs and capping out-of-pocket expenses for individuals with chronic health problems would indeed be big improvements. But a lot of details about her plans are still lacking, and her focus on “value” of care could mean a great deal, or nothing much. It certainly won’t inspire voters, whereas Sanders’ call for “Medicare for all” has been testing well for a long time.

Which is where that slogan comes in. What it means, translated to the retail-politics level, is, “The best defense is a good offense.” Progressive Democrats, including Sanders, have learned this in the past couple of years, notably with their proposals to expand Social Security—proposals that play well with the public, despite which Clinton, for one, has studiously not endorsed them. Why would it not be the case that the best defense of the ACA is to go farther in a direction it probably has to go anyway?

Aaron himself implicitly accepts this argument:

Let me be clear: We know that high-quality health care can be delivered at much lower cost than is the U.S. norm. We know because other countries do it. In fact, some of them have plans not unlike the one Senator Sanders is proposing. We know that single-payer mechanisms work in some countries. But those systems evolved over decades, based on gradual and incremental change from what existed before. That is the way that public policy is made in democracies.

Right—but then the question is, When do we get started? When does the Democratic Party get out of its defensive crouch? It’s been six years since ACA passed, and there’s little sign of change. The New Deal Democrats were already making major changes to Social Security, including replacing its basic funding mechanism, less than four years later. Nothing comparable is happening with ACA today. That’s a disturbing and dangerous trend; every year that nothing’s done to improve the structure of the program, make it more generous, and better address participants’ needs, is a victory for its enemies.

Sanders’ proposal is a hard push in the direction Obamacare must take if the goal of universal, affordable health care is to survive—which is to move past the jury-rigged structure of exchanges and Medicaid expansion to one that wields real economies of scale and that everyone can participate in. Sadly, too few Democrats, perhaps afraid of a “Gotcha!” from the Wall Street Journal editorial page, are ready to acknowledge this.

Aaron, for example, raises some reasonable questions about Sanders’ plan: What would happen to special facilities for veterans and service members’ families? How realistic are his assumptions for cost savings? How will the package of services covered be decided upon? Would hospitals go bankrupt? Would we have a shortage of doctors and nurses? Yet he asks them as if not to search for answers, but to demonstrate that there are no answers—just as Republicans have been doing with regard to Obamacare since it was embryonic.

This is not good politics, especially now. Presidential campaigns are the time to unveil grand visions, not to bore the electorate with small-ball proposals. To take a couple rather grim examples, Ted Cruz probably won’t get a chance to find out if he can make Mideastern deserts glow, and Donald Trump probably can’t bar all Muslims from entering the country. But they want to take us in those general directions, which, for their followers, is the point. In a more sane context, some establishment figures on the Democratic side understand this about Berniecare as well. Says Daily Kos’s Joan McCarter:

Here’s where Sanders and Clinton should agree. Expanded Medicare needs to be included in Obamacare. We need the public option added to it. That’s an interim fight that would definitely be worth both campaigns’ time. Indeed, the may lose the whole health care ball game.

Of course, that’s not an “interim fight.” The public option is the heart of the system that ACA needs to evolve into. If Democrats dismiss Sanders and continue to play defense, however, they’ll never get there. Indeed, they could lose the whole health care ball game.