Here’s something really scary for Halloween: the plan being pushed in the budget super committee by Alice Rivlin, Alan Simpson and Erskine Bowles to cut Social Security benefits by changing the way inflation is measured. Any member of Congress who goes along with this plan will deservedly be as popular as a vampire at a blood drive.

Retirees living on Social Security are mostly just scraping by. The average retirement benefit is only about $14,000 a year in 2009, and most retirees depend on Social Security for half or more of their incomes. Knowing how tight their budgets are (and their proclivity for voting), Democrats and Republicans alike have promised not to cut the benefits of people nearing retirement, not to mention the benefits of people who have already retired. Yet the only way the inflation measure can reduce the deficit over the next 10 years is by cutting Social Security cost-of-living adjustments for current and near retirees.

The members of Congress who want to make this benefit cut don’t want to admit they’re breaking their promises to retirees. So they disguise the cuts as a technical change—an improvement in the cost-of-living measure. That’s hogwash. The alternative index they propose for the Social Security COLA is not an improvement over the current measure; it’s almost certainly a worse indicator of the rising cost of living faced by seniors. And there’s nothing technical about its expected effect on retirees’ checks. The COLA reductions it will cause will cost the average retiree about $1,700 a year by 2031.

Social Security’s annual cost-of-living adjustment is based on the Consumer Price Index for Urban Wage Earners and Clerical Workers, or CPI-W. Ironically, the CPI-W measures changes in the cost of living for workers, excluding retirees and other Social Security recipients who aren’t in the labor force. This measure doesn’t accurately reflect the cost of living for seniors. Seniors have experienced higher inflation because they spend a greater share of their incomes on out-of-pocket medical expenses, and health costs have risen faster than overall inflation in recent decades. An index that specifically tracks the cost of living of seniors has risen roughly 0.27 percentage points faster per year than the CPI-W.

The rationale for the change the super committee is contemplating is that the current price index overstates inflation because it doesn’t fully account for the ability of consumers to change their buying habits in response to price changes. In other words, if the price of oranges goes up, people will buy more apples and fewer oranges, and this change isn’t fully reflected in the CPI-W even though the consumption “basket” evolves over time to put more weight on apples and less on oranges.

The problem with this argument is that it doesn’t look at the growth in the costs actual retirees face over time. Not only are seniors harder hit by escalating medical costs than the working-age population, but since they have roughly half the household incomes, they spend a greater share on necessities like rent and utilities. It’s likely that the CPI change advocated in the super committee will understate inflation in the goods and services the elderly mostly purchase, and it may actually overstate their ability to change consumption habits in response to price changes. No one disputes that it will lead to benefit cuts that start small but compound over time.

Benefit cuts are justly unpopular across the political spectrum—especially cuts that affect retirees and near-retirees. But Republican members of Congress have a double problem. The CPI change would affect income taxes, too – not just Social Security and veterans’ benefits. How does anyone who took a no-tax-increase pledge defend voting for a “technical change” that will raise $72 billion in taxes by 2021 on tens of millions of Americans? They might be tempted, since there will be nearly $2 of Social Security cuts for every $1 of increased tax revenue. But at the end of the day a vote for the CPI change will feed the disgust of Tea Party types as much as progressives and liberals.