The first time the Supreme Court took a case on Obamacare, most supporters of the law responded with derision. Who could take seriously the argument that the “individual mandate” was unconstitutional? In fact, the conservative Supreme Court majority could—though Chief Justice John Roberts ultimately spared the law by reconstruing the penalty for failing to comply with the mandate as a tax within the power of the Congress to impose.

This time, however—now that the Court has decided to hear a challenge to the subsidies available for insurance purchased on the federal exchanges—the reaction among supporters has been different. “Panic” might go a little too far—until you reckon in the equally urgent calls among supporters for everyone not to panic.

Why the difference? Partly because of the near-death experience last time, no doubt. Another reason may be the bad conscience that comes of insisting the problem is really just a “typo.” (A drafting error of the first magnitude, it may be; a policy mistake that was insufficiently considered and got in because of the haste and chaos, it may be; a typo it is not.) But in the broadest sense, the problem is just how fragile Obamacare has proven to be for so long now—and the worry about what the legacy of this presidency looks like without it. 

Whatever else may be true of the Affordable Care Act, it is unique in the modern history of truly major legislation in that support for it in Congress collapsed almost from the moment of its enactment. We all recall the tortuous path to final passage. Republicans had already checked out of the process en bloc. Then came the surprising victory of Republican Scott Brown in Massachusetts to fill the Senate seat of the late Ted Kennedy, which suddenly deprived the Democratic majority in the Senate of its ability to defeat a filibuster. That in turn eliminated the possibility of a conference committee to reconcile House and Senate versions of reform, the golden opportunity to scrutinize the emerging legislation and work out the kinks.

Instead, to move forward, the House had no choice but to pass the Senate legislation despite grave misgivings among supporters there. This it did narrowly, followed by a hastily-crafted bill straightening out some of the policy and finances of the reform effort, which the Senate could take up as a budget matter under the “reconciliation” process, which disallow a filibuster, thus requiring only a simple majority.

Yes, this was  “a big f—ing deal,” as Joe Biden was caught on mic remarking to a triumphant Obama. Democrats at the time believed public opinion on Obamacare would inevitably swing in their favor. No one foresaw the loss of the House in 2010, the continuing unpopularity of the law, the horribly botched rollout of healthcare.gov, the sinking approval ratings of the president after his reelection, and the loss of the Senate in 2014. I think it’s reasonable to speculate that Democrats’ congressional fortunes would not subsequently have declined as sharply if the effort to pass the Affordable Care Act had collapsed. But even so, the impression lingers that Democrats would never have forgiven themselves for failing to cross the finish line on health care reform in 2010.

The problem is that there is no fixing the Affordable Care Act—or rather, there is no legislative adjustment to it available. It might as well have been carved in stone. If, somehow, a health care reform bill could have passed with significant bipartisan support, then likely it would have been possible in the years since for congressional majorities to adjust it as necessary, with a “technical corrections” bill here and a rider there—even with a change in control of Congress. That’s impossible now. Democrats don’t have the power, and the GOP has no intention of helping Obamacare out. 

Hence the heavy lifting of the executive branch in trying to make it work, with such decisions as the delay of the employer mandate and the continuation of high-risk pools that were supposed to be dissolved when the exchanges opened. Hence the John Roberts reconstruction of the legislation turning the penalty for failing to comply with the individual mandate into a tax. Even worse, everybody knows that there was no way the bill could have passed with this “tax” provision labeled as such. And now here we go again: Roberts and the Supreme Court will either have to fix a glaringly obvious problem in the black letter of the law on their own, or—yikes. 

And that’s where the sense of fragility comes from. Democrats like to say that Obamacare is a done deal, and everybody should move on. Except that it’s still very much a work in progress, and the progress that takes place does so against the backdrop of a unified opposition party, a skeptical public, and a Supreme Court that has issues of its own, about which we will no doubt find out more soon. And then, of course, there’s the unthinkable: What if the GOP captures the White House in 2016? Short answer: Obamacare is dead. And that would be its own “big f—ing deal” for the legacy of a presidency that is now seriously troubled.

There is, however, a plausible longer answer to the question of what happens to Obamacare under a GOP-controlled Congress and White House. It’s that “repeal and replace” turns out to mean “reform and rebrand,” the result of which would really be the return of Obamacare to the legislative process. The Supreme Court (though I doubt it) might even choose to try to kick that return off sooner by ruling that Congress screwed up subsidies in the federal exchanges and only Congress can fix the problem, thus creating a policy crisis.

In the long run, an accommodation between the parties on health care, subject to future legislative reform initiatives from left or right as politics dictates, seems inevitable. There’s a lot of this story that has yet to happen, of course. But Obama’s supporters should relax, because at that point, his role as catalyst will indeed be his principal presidential legacy.