In about a month, if nothing is done, the federal government will hit its legal debt limit. There will be dire consequences if this limit isn’t raised. At best, we’ll suffer an economic slowdown; at worst we’ll plunge back into the depths of the 2008-9 financial crisis.

So is a failure to raise the debt ceiling unthinkable? Not at all.

Many commentators remain complacent about the debt ceiling; the very gravity of the consequences if the ceiling isn’t raised, they say, ensures that in the end politicians will do what must be done. But this complacency misses two important facts about the situation: the extremism of the modern G.O.P., and the urgent need for President Obama to draw a line in the sand against further extortion.

Let’s talk about how we got here.

The federal debt limit is a strange quirk of U.S. budget law: since debt is the consequence of decisions about taxing and spending, and Congress already makes those taxing and spending decisions, why require an additional vote on debt? And traditionally the debt limit has been treated as a minor detail. During the administration of former President George W. Bush — who added more than $4 trillion to the national debt — Congress, with little fanfare, voted to raise the debt ceiling no less than seven times.

So the use of the debt ceiling to extort political concessions is something new in American politics. And it seems to have come as a complete surprise to Mr. Obama.