So what happens if the exchanges aren’t ready on time? Or what if the exchanges open—but there are big problems with enrollment? The exchanges, which are intended to both allow individuals to select insurance plans and determine eligibility for the law’s subsidies, are the primary vehicle for individual compliance with the mandate. Which means that if even a handful of the exchanges—remember, there’s one for every state—aren’t open or aren’t broadly functional, then large numbers of people are likely to have trouble complying with the individual mandate.

And if that happens, there will be an awful lot of pressure to repeal, or at least delay, the mandate. Indeed, it’s the kind of pressure you can plausibly imagine pushing Democrats to vote against the requirement. Remember, many Democrats were wary of the mandate. As a White House candidate in 2008, President Obama openly opposed it. If complying with the mandate is difficult enough, then it’s possible—maybe not likely, but possible—to imagine Democrats agreeing to strike the provision, or delay its implementation.

And what then? Well, Democrats have (probably correctly) pointed out that the law’s insurance industry reforms don’t really work without a mandate to ensure that most everyone buys in. But here’s the thing: If the mandate gets taken down under the above scenario, it’ll be because the law isn’t working with the mandate either.