WE have two political parties in America, runs a saying that conservatives like to quote. One is stupid, the other is evil. And when they join forces to do something that’s both stupid and evil — well, that’s what we call “bipartisanship.”

The payroll tax holiday that passed Congress in the winter of 2010 was a rare exception to this pessimistic rule. Cutting the payroll tax was good short-term politics for both Democrats and Republicans: it was a tax cut that liberals hoped would double as stimulus, and a boost to the middle class that conservatives could support without embracing new federal spending. But more important, it opened the door to what would be good long-term policy as well — because more than almost any feature of the American tax code, the payroll tax deserves to be pared away into extinction.

But now Washington is in danger of practicing payroll-tax bipartisanship of a more destructive sort. While the White House and Congressional Republicans wrestle over where to set income tax rates and how and whether to cut spending, the payroll tax holiday has been orphaned. Lacking noisy champions and press attention, it’s in danger of expiring at the end of the year out of political indifference.

That outcome would be unfortunate. Payroll taxes are a relic of New Deal Machiavellianism: by taking a bite of every worker’s paycheck and promising postretirement returns, Franklin Roosevelt effectively disguised Social Security as a pay-as-you-go system, even though the program actually redistributes from rich to poor and young to old. That disguise has helped keep Social Security sacrosanct — hailed by Democrats because it protects the poor and backed by Republicans as a reward for steady work.