For people who claim that defaulting on U.S. debt is unthinkable, President Obama and Harry Reid are sure behaving as if they can't wait for that day to arrive. Why else are they refusing to take up the multiple Republican offers to guarantee that the U.S. will never default?

Treasury Secretary Jack Lew now says his department can't be sure it will have enough cash to guarantee interest payments after October 17, and he is raising alarms about a default catastrophe. Mr. Obama declared again on Wednesday that he'll only negotiate with Republicans on taxes and spending if they first raise the debt limit without conditions—so dire would the damage from default be to the U.S. economy and reputation.

House GOP leaders also insist they don't want a default, and they've already passed a bill to prevent it—not that the media have paid any attention. First sponsored in 2011 by California Republican Tom McClintock and Pennsylvania Senator Pat Toomey, the Full Faith and Credit Act is essentially an insurance policy against miscalculation. Their bill certifies that U.S. sovereign debt will always be repaid, on time and in full.

If Congress fails to authorize a statutory increase in the debt limit—during a period of, say, intense political conflict over the fisc like the one now—the bill stipulates that Treasury can continue making contractual interest and principal payments to bond holders and rolling over debt with incoming tax payments. Debt service gets the first call on revenue.

The McClintock-Toomey bill replicates the guarantees that state constitutions have had for hundreds of years to strengthen investor confidence. It gives the Treasury Secretary discretion to prioritize among other federal obligations until the political deadlock ends, tempers cool and the parties can reach a deal. But it makes his first priority to protect the full faith and credit of the U.S.