The way you get a law passed (or repealed) in the U.S. is, after all, pretty well laid out in the Constitution: you need a majority in both houses of Congress, and you need a President who will sign the bill. If the President vetoes it, you need a supermajority to override the veto. Obamacare is the law of the land because it cleared these hurdles.
Now Republicans would like to repeal Obamacare. But they don’t have the votes. That’s the simple reality that all the talk about the unpopularity of Obamacare is meant to occlude: Republicans control only one house of Congress. In the past, that would have been enough to end the story. Republicans would have voted to repeal the law (as the House has done innumerable times already), failed to get repeal through the Senate, and moved on, biding their time and waiting for the day they took back the Senate and the Presidency. Self-evident as it sounds, that’s how democracy is supposed to work: you need a majority in order to change the law.
What the Republican hard-liners have decided instead is that even though they don’t have the necessary votes they should still get their way, and, in order to accomplish that, they’re going to hold the economic well-being of the country hostage. This is a radical and far-reaching demand, and acceding to it would in effect mean that controlling one house of Congress would be all you need to set policy. (Had congressional Democrats followed the Republican playbook in the nineteen-eighties, when they controlled the House but not the Senate, they would have demanded a repeal of the Reagan tax cuts in exchange for voting to raise the debt ceiling.)
There’s no obvious place where this process would stop, since there’s no distinct connection between Obamacare and the national debt. (At least the 2011 showdown was over fiscal issues.) Any law that hard-liners wanted to repeal (or pass), from abortion to gun control, could be pegged to a debt-ceiling vote. The ability to push through legislation would no longer be about which party was better at winning elections; it would be about which party was more willing to take the government, and with it the economy, over the cliff. Indeed, if the Republicans were to win this standoff, the result would be far more general uncertainty, since every debt-ceiling increase would become an occasion for rewriting laws.