And if we don't lift that self-imposed limit, we enter a world of economic pain. We couldn't borrow more to pay for the spending Congress has already authorized. We'd have to pay what we owe out of tax receipts and rolled-over debt instead. But as the Bipartisan Policy Center (BPC) points out, tax receipts and rolled-over debt would only cover 68 percent of our bills. So we'd have to cut the rest. This immediate 32 percent austerity could hit anything from Social Security checks to food stamps to the debt itself. Now, this last one is unlikely, but not impossible. How unlikely depends on whether the Treasury can prioritize its payments. Let's think about what would happen if it can and if it can't.

Scenario #1: Treasury can prioritize the debt, so we get austerity but not default. Suppose the government doesn't pay its bills as they come due. Suppose instead it pays back the debt before anything else. In that case, interest rates wouldn't rise -- they'd fall. At least longer rates. Now, short-term term rates on debt due to be paid back right after the debt ceiling hits would rise. But that's it. After all, investors flee to the safety of U.S. government debt (which pushes down rates) anytime things look grim.

And cutting 32 percent of federal spending overnight would certainly make things look real grim real fast. Indeed, that much austerity would cost us millions of jobs at an annualized pace. If a debt ceiling showdown somehow lasted much more than a few days, the economy's heart palpitations would turn to cardiac arrest -- which, as long we were still paying the interest on it, would mean much more demand for our debt. It's paradoxical, but threatening to default on our debt might make investors want our debt more than ever. You know, as a safe haven.

But this optimistic case isn't much of one. Even forgetting the austerity. For one, the spectacle of House Republicans once again debating whether or not to light the fuse on an economic bomb would be enough to crater consumer confidence. As Betsey Stevenson and Justin Wolfers point out, that's what happened the last time Republicans weren't so sure we should pay our bills back in 2011 -- and it took households awhile to get over their shock. For another, introducing any possibility of defaulting on the debt would be enough to make the financial system seize up.

But wait. Isn't prioritizing payments supposed to mean we pay all the interest on the debt on time? Why would markets still worry about a default? Well, something might go wrong. And if it did, the financial system would not be in good shape. In fact, just the risk that it might is enough to make the financial system in not-so-good shape already. That's because our debt isn't just debt; it's money, too. Or at least close enough to money that we can say it's "money-like."