Medicare now faces a more uncertain future than at any time in its history. That’s not because it has lost popularity or failed to control costs as effectively as private insurance has. On the contrary, the program continues to enjoy overwhelming public support, and since the late 1990s, its costs per beneficiary have grown more slowly than those of private insurers. Nor does Medicare confront an imminent crisis; in fact, its costs have decelerated in the past year.

It is one of the anomalies of today's politics: The party that professes absolute fealty to the Constitution in its original form is also the most eager to change it. Exhibit A is the amendment pushed by Republicans to require a balanced budget every year, cap federal spending at 18 percent of gross domestic product, and bar any increase in taxes without a supermajority of two-thirds of Congress or any increase in the national debt without a supermajority of three-fifths.