I wrote last week that Bernie Sanders's campaign had changed my mind and convinced me that he is right to set tuition-free college at public universities as the goal for higher education policy.

But I also noted that I wasn't sold on the implementation details of his plan, and that in my experience Sanders enthusiasts were not especially aware of the content of his plan.

Big-picture principles are important, but implementation is important too. It's worth reading Sanders's actual plan, since not only is there a lot of nitpicking one could do but there's also an enormous glaring flaw. It pretty clearly wouldn't achieve its goal of making the United States a country where students pay zero tuition to attend public colleges.

This matters because it's not so much a design flaw as a concession to some practical realities that Sanders doesn't admit to on the campaign trail — realities that his supporters would be well-advised to pay more attention to.

So even if the idea of free college is exciting, there are a lot of reasons to doubt it would ever happen.

Bernie's plan: matching funds for states that want to eliminate tuition

Sanders's own summary of his College for All Act makes it pretty clear that the act would not, in practice, eliminate college tuition. What it would do instead is offer federal matching funds on a 2-to-1 basis to states that want to increase higher education spending in order to eliminate tuition:

This legislation would provide $47 billion per year to states to eliminate undergraduate tuition and fees at public colleges and universities. Today, total tuition at public colleges and universities amounts to about $70 billion per year. Under the College for All Act, the federal government would cover 67% of this cost, while the states would be responsible for the remaining 33% of the cost. To qualify for federal funding, states must meet a number of requirements designed to protect students, ensure quality, and reduce ballooning costs. States will need to maintain spending on their higher education systems, on academic instruction, and on need-based financial aid. In addition, colleges and universities must reduce their reliance on low-paid adjunct faculty.

There are two relevant things to note here.

One is that the 2-1 match for eliminating tuition is much less generous than the 9-1 match offered by the Affordable Care Act's Medicaid expansion, yet many states have chosen not to expand Medicaid.

The other is that because the College for All Act requires qualifying universities to reduce reliance on low-paid adjunct faculty in addition to eliminating tuition, in practice the federal match is worth even less than 2 to 1.

Between the years 2008 and 2015, 47 out of 50 states chose to cut higher education spending.

Several states have moved to partially reverse those cuts over the past year or two. And with federal matching funds on the table, several solidly blue ones would likely try to take advantage of Sanders's offer. But this simply isn't many states. After the electoral disasters of 2010 and 2014, Democrats don't even have solid control over states as blue as New York, Illinois, Massachusetts, and Maryland — to say nothing of places like Michigan, Ohio, or Florida that are under complete Republican control.

Put it all together, and what Sanders has is a plan for tuition-free college in Vermont and, if he's lucky, California, but not for the United States of America.

Sanders's plan acknowledges path dependency

One possible reaction to this would be to slam the plan as inadequate and call for a more ambitious scheme to federalize public higher education financing in the United States. That, after all, would be the only way to guarantee the high-level result that Sanders is promising. It also seems like a reasonable idea in principle. There's no particular reason a poor kid growing up in Alabama should have his access to higher education constrained by the limited financial resources and stingy right-wing politics of his home state.

But Sanders has good reason for not proposing that.

US higher education policy in 2016 is not a blue-sky landscape. We can't turn back the clock to the 1860s and launch federal higher education. The country is full of established public colleges and universities that operate primarily under state auspices. In other words, college in America is an example of path dependency.

A proposal that massively disrupts the governance and financing at all of the public higher education institutions in America would be a recipe for massive backlash. Even though there's nothing wrong in principle with federally managed higher education, and even though there are plenty of countries around the world where it works fine, these institutions are resistant to such massive change.

Sanders should acknowledge path dependency elsewhere

This leaves us with two problems.

One is unrealistic expectations. People who are voting for Sanders because they are enthusiastic about the idea of free college should know that even if his political revolution fully sweeps Washington, college tuition still isn't going to be eliminated unless governors and state senators can be convinced to raise higher education spending.

The other is selectivity. Sanders's plan does not truly deliver tuition-free college for the exact same reason that the Affordable Care Act does not truly deliver universal health care: path dependency and reliance on state government cooperation. If all 50 states had governments in place that wanted to achieve universal insurance coverage, then the Affordable Care Act creates a framework in which they could easily and affordably do so. Indeed, it provides much more of the necessary financial resources than does Sanders's College for All Act.

Great ideas need down-ballot wins

Sanders's campaign was initially interpreted by most people, myself included, as a protest candidacy whose purpose was to raise issues and talk about ideas.

And on that level, it's been very effective — reminding Democrats of the shortcomings of Obamacare, introducing the free college concept to a national audience, and elevating concerns about bank regulation and globalization that would have gotten short shrift in a Hillary Clinton coronation.

But examining the details of Sanders's higher education plan is a reminder that there's relatively little reason to think that replacing Barack Obama with a more left-wing president would be the major difference maker on the issues that Sanders and his supporters care about. On most issues — including both extension of insurance coverage and funding of public higher education — the proximate barrier to more progressive policy is in the statehouse or the House of Representatives, not the White House.