Senator Bernie Sanders unveiled his single-payer "Medicare For All" plan today, and so far he's already got 15 Democratic co-sponsors. More may soon add their names to this list. The plan is -- as any such plan has to be, right now -- merely aspirational, since there is no way it will ever even get a Senate floor vote with Republicans controlling the chamber. Bernie's bill also punted completely on the crucial questions: "How much will it cost, and how will we pay for it?" -- something many will take him to task for (as indeed I already pre-emptively did). But even having said all of that, Bernie's Medicare For All bill sets a very important marker for the debate to come, whether that debate takes a few years or longer. Because Bernie has -- with his bill and also with his relentless championing of the issue both during the 2016 campaign and since -- successfully moved the "Overton window" on single-payer healthcare in the United States.

Before I explain that further, let's take a quick look at what Bernie's Medicare For All plan would include, from a good rundown in the Washington Post:

• It establishes an (almost) true single payer system in which private and employer-based insurance is replaced by an expansion of Medicare to include nearly every American.
• It does so over four years, lowering the Medicare eligibility age to 55 and insuring all children in the first year, then expanding the plan to include all citizens over the following three years.
• It includes areas of coverage not currently offered by traditional Medicare, including dental, vision, and hearing aids.
• It eliminates nearly all co-pays, coinsurance, and deductibles.
• It allows the expanded Medicare system to negotiate drug prices, which Medicare is currently forbidden from doing.
• It does not include nursing home coverage, which would continue to be covered under Medicaid, nor does it eliminate the VA or the Indian Health Service.
• It would pay providers at current Medicare rates, which are lower than private insurance but higher than Medicaid.

That is pretty sweeping, and if enacted could wind up being a plan Americans would be proud and happy to have. Especially that part about including dental, vision, and hearing aids -- which are (by any rational definition) part of a person's healthcare, but for some bizarre reason are treated totally separately by the U.S. insurance marketplace. Also important is the elimination of co-pays and deductibles, which will be widely supported by consumers. Bernie has completely redesigned the American healthcare system, and offered up one he thinks would be cheaper, better, and easier for all Americans to deal with than what they've got now.

Of course, as I mentioned, this bill in its current form is not going to pass any time soon. But it will immediately become the benchmark all other progressive changes to the system will be measured against. That in and of itself is a valuable contribution to the debate.

But back to the Overton window. Joseph Overton came up with this concept to describe how there is a "window" in politics of what is considered politically viable. He posits a spectrum of ideas between "more free" and "less free" to describe government's role, which he intentionally placed on a vertical axis to avoid the inevitable "left/right" comparisons. Since it was introduced, however, the concept has indeed been adopted to the left/right, liberal/conservative political spectrum. But whether you're talking up or down, or left or right, the basic concept remains pretty easy to grasp.

The spectrum's center is labeled "policy." This is where current policy exists on any subject. Moving away from the center, in both directions, is a scale of viability:

• Policy
• Popular
• Sensible
• Acceptable
• Radical
• Unthinkable

New ideas move from the outer edge of the scale (unthinkable) to the center, if they turn out to be successful politically. The Overton scale charts how ideas progress during this process. Once an idea hits a certain level, it lands within the overall Overton window and is considered politically viable.

Single-payer healthcare started squarely in the "unthinkable" category, even for most Democrats. It was some wild-eyed lefty idea that would never become reality, or even be politely discussed among "Serious People" in Washington (when they hold their cocktail parties). As I pointed out earlier this week, it was only eighteen months ago that Hillary Clinton dismissed Bernie Sanders's support for single-payer as "a theoretical debate about some better idea that will never, ever come to pass." For Clinton, at the time, single-payer was definitely in the unthinkable category.

That wasn't that long ago, it bears pointing out. Already, the idea has moved up one notch, to "radical." By definition, people (even "Serious People") are indeed thinking about it and offering up solid proposals for its implementation. More and more of the general public now agrees that the idea is actually "acceptable," foreshadowing the next step on the Overton scale.

Single-payer still has a few steps to go, of course. But Bernie's plan may move it along the scale a lot faster than it would have without his Medicare For All bill. Remember, it wasn't that long ago that the Democratic Party as a whole considered the idea unthinkable. Now, though, future movement to acceptable, sensible, and popular seems almost within reach.

There's an easier way to put this, actually. The quote is often misattributed to Gandhi, but was actually first said by a union leader named Nicholas Klein, in a speech given in 1918 to the Amalgamated Clothing Workers of America:

And, my friends, in this story you have a history of this entire movement. First they ignore you. Then they ridicule you. And then they attack you and want to burn you. And then they build monuments to you.

This has been shortened even further, to the succinct slogan: "First they ignore you, then they laugh at you, then they fight you, then you win." On this much simpler political scale, Bernie Sanders has personally moved the single-payer debate from being ignored through being laughed at to the point where we find ourselves now. Bernie's Medicare For All bill will be fought, ideologically. Conservatives are already reportedly gleeful at the prospect of tying all Democrats to Bernie's bill in attack ads during upcoming campaigns. They won't be the only ones fighting Bernie's bill, either, since the entire health insurance and prescription drug industries will devote millions of dollars to the fight against single-payer as well.

Success is in no way guaranteed. But then it never is, for new political ideas. The fight will be long and brutally hard, win or lose. But that doesn't mean (as most Democrats would have said not so long ago) that it isn't a fight worth having. The Patient Protection and Affordable Care Act ("Obamacare") was essentially a conservative idea, after all. It preserved the health insurance marketplace in a manner originally laid out by a member of the conservative Heritage Foundation, and it was first implemented on a statewide basis by Mitt Romney, after all. It certainly has its flaws, but it also succeeded in moving America a giant step forward in how we think about healthcare. The Overton window on "health insurance should be universally available to all" has already moved much further than the single-payer concept, and Obamacare certainly started the conversation. But Obamacare was never seen by idealists on the left as the end of the road. Bernie Sanders has now unveiled his version of where that road should ultimately lead. We'll probably have to go through some interim steps (like a "public option") before we get there -- if we ever do. But Sanders has now provided a clear and comprehensive goal to shoot for in the end.

So let's have this fight. Let's begin the single-payer discussion in earnest. Let's move the Overton window until even Republicans begin to see how popular the idea could be. Medicare For All has just shifted from being a slogan yelled at Sanders rallies to being a concrete proposal that even Serious People at their D.C. cocktail parties will have to now confront. That is progress. Who knows where the debate will be centered by the time the 2020 presidential primary season rolls around?

-- Chris Weigant

Cross-posted at The Huffington Post

Follow Chris on Twitter: @ChrisWeigant