In a polity as unwieldy and heterogeneous as ours, attaining 70 percent public approval for any policy is a minor miracle. Doubly so when it’s a fairly esoteric provision of healthcare reform and, crucially, when it involves vital progressive policy. Yet in the past few weeks, three successive national polls have shown that roughly 70 percent of respondents favor a Medicare-style public plan as part of healthcare reform.

It makes sense. Almost every American knows someone who uses Medicare, and the satisfaction with that program is famously high. From a policy perspective, a public option would serve an essential purpose: if it performed with the efficiency and cost control of Medicare, it would impose discipline on private insurers through competition. In other words, it would set a kind of baseline of care by giving people a choice.

Most Democrats in the House and Senate seem to be on board, as is President Obama, who not only campaigned on a public option but took quite a bit of time to make the case for it during a recent White House midday press conference and later ABC prime-time town hall meeting. “If private insurers say that the marketplace provides the best quality healthcare, if they tell us that they’re offering a good deal,” he said during the press conference, “then why is it that the government–which they say can’t run anything–suddenly is going to drive them out of business? That’s not logical.”

Those who oppose the public option are the very powerful defenders of the status quo. First, there’s the Republican Party, which, as in 1993, is dead set on killing (or, failing that, maiming) any comprehensive healthcare reform. Partly this is because of ideology; mostly it’s a matter of politics: allowing Democrats to deliver much-desired reform would be conceding a major, perhaps epoch-shaping defeat. Then there is the health-industrial complex, from the American Medical Association to the insurance companies, which are quite profitable, thank you very much, and have no interest in being forced to compete with a plan that would undoubtedly have lower administrative costs. Indeed, despite their avowed reverence for competition–which they claim a public plan would undermine–insurers in large parts of the country enjoy a near-monopoly. Health Care for America Now recently issued a report showing that 94 percent of local insurance markets are “highly concentrated,” according to the guidelines used by the Justice Department.

Though this is a winnable fight, the outcome is up for grabs. Key Senate Democrats are wavering in the face of pressure from the healthcare industry: Kent Conrad of North Dakota is skeptical of a public option; North Carolina’s Kay Hagan has come out against it; Ben Nelson of Nebraska has gone back and forth too many times to count–although, hearteningly, he has proven vulnerable to pressure from progressives. Arlen Specter has come out in favor of a public option, and there are at least a few Republicans who seem open to it.

A public option is needed but is by no means sufficient, and progressives will have to be vigilant about other key parts of the reform package, such as the income level at which healthcare is subsidized. But politically the public plan is a useful and worthy rallying cry. The president, while forcefully advocating for the public option from the bully pulpit, has also said there are no “lines in the sand.” It is the job of citizens, then, to draw them.