Now that the Supreme Court has left the fate of the Affordable Care Act hanging another few days, it seems an opportune moment to pose a question that has been growing on me after several recent reporting trips: why aren’t the most obvious beneficiaries of the law more aware of it?

There is an assumption in much national coverage of the law’s unpopularity that the right has won the “messaging war” on the law, partly, as Abby Goodnough lays out in a strong New York Times piece today, by spending far more to bash it on the airwaves than the law’s supporters have to defend it. There’s also an assumption that many in the white working class who will benefit from the law are sharply opposed to it because they, well, don’t much care for the guy who signed it into law. Both of these things are no doubt true. But my reporting leads me to think that the problem, to a large extent, gets to a very specific issue: the decision by the administration not to broadcast the part of the law that will have the most obvious, immediate impact on the working class: the expansion of Medicaid.

At least half to the expanded health coverage in the law—an estimated 16 million people—is to come through bringing Medicaid eligibility, in 2014, up to a national threshold, 133 percent of the poverty level (about $31,000 for a family of four.) The impact is going to be especially big in states of the South and West that now have exceedingly stingy eligibility rules—in Texas and Virginia, among others, earning as much as $10,000 a year disqualifies even a parent of small children from getting covered; if you’re childless, forget about it.

But when was the last time you heard the administration talking up this massively consequential part of the law? To the extent it does try to promote the law, it’s all about the filling of the donut hole for the Medicare drug benefit, letting young people stay on their parents’ insurance until age 26, credits to small businesses, and new rules on preexisting conditions. All important elements, but none as significant as the Medicaid expansion.

Well, it’s not hard to understand why the Medicaid part of the law was played down. It’s a pure expansion of the public safety net. Whereas the rest of the coverage expansion is, as Obama likes to stress, based on the private sector—by providing subsidies for people further up the income ladder to purchase private coverage—the Medicaid expansion is simply adding a lot more people to the rolls of a government insurance program (although in many states, Medicaid is contracted out to private HMOs, an increasingly lucrative sector for insurers). Moreover, emphasizing the Medicaid expansion would only draw more attention to the bewailing by governors (mostly Republican, though not all) about the burden that the expansion will add to their budgets—never mind that the cost of the expansion will be picked up almost entirely by the federal government, an outrageously good deal for the aforementioned “stingy” states that will be paying far, far less to cover the working poor in their states than the more generous states will be paying for their share of Medicaid costs.