Tucked into Republicans’ tax overhaul bill is a technical tweak to how inflation is measured. The change is designed to hold down the deficit, but over time it becomes a significant tax increase that hits many of the same middle-class households who start out as the plan’s beneficiaries.

Both the House and Senate versions of the tax bill would adjust the income thresholds at which tax rates change using a “chained” measure of inflation rather than the better-known consumer-price index.

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