“WE ARE NOT going to reduce the deficit or subsidize tax cuts for the rich on the backs of America’s seniors and working families,” House Minority Leader Nancy Pelosi (D-Calif.) decreed Friday. “No benefit cuts in Medicare and Social Security.”

The first part of that vow is reasonable. The second is irresponsible. Of course, any grand bargain on the debt must be balanced. As we have said innumerable times, it cannot be accomplished through spending cuts alone, either in discretionary programs or entitlements. Tax revenue — not simply loopholes closed but new money, and lots of it, about $1 trillion worth — is required. Much of that should come from those who have prospered most as income inequality has widened. To the extent that the liberal Democratic position is not whittling away a widow’s Social Security check to pay for a millionaire’s tax break, sign us up. But a vocal segment of the party, particularly among Ms. Pelosi’s fellow House members, is drawing a different line in the sand: no benefit changes whatsoever. That is not a tenable position.

When it comes to Social Security, the no-benefit-cuts caucus argues that the program is not the major driver of current deficits. This is correct but irrelevant. The program is on a course to run out of enough money to pay promised benefits in 2036. The longer policymakers wait to make adjustments the more painful these adjustments will be — and the more risk they pose to the most vulnerable beneficiaries that Democrats assert they want to protect. Fixing Social Security is important for its own sake; it is going to have to happen sooner or later. If the debt ceiling debate presents a sensible occasion for getting that done, at least in part, so much the better.

The question then becomes whether scheduled benefits are off-limits, as liberal Democrats insist, and therefore any necessary adjustments must take the form of higher payroll taxes or other means of injecting more revenue into the system. Here, again, balance is called for. Insisting that every future Social Security recipient must be paid every cent of benefits now promised is unnecessary and unfair. Indeed, talking about cutting benefits is fundamentally misleading. Because of the generous way that payments are calculated, future retirees are set to receive far larger checks than current ones — even after accounting for inflation. The only question is how much more they will get. This is a particularly reasonable question since Social Security, as Eugene Steuerle of the Urban Institute points out, “has morphed into a middle-age retirement system,” with typical beneficiaries spending one-third of their adult years collecting benefits.

The specific benefit changes under discussion, such as changing to a more accurate measure of inflation, can be debated. Would the new inflation measure take into account higher health costs for seniors? Should it be adopted in conjunction with extra protections for the poorest and oldest seniors? Fair questions, but the no-cut response is not a fair answer.

Insisting on no change to scheduled benefits for any retiree, no matter how affluent, ignores the twin imperatives of protecting less affluent seniors and ensuring that society retains enough resources for programs for the non-elderly. It is not a responsible position — nor, at bottom, a progressive one.