Among many faults of a US Senate proposal to repeal the manufacturer's tax deduction for large oil companies, the worst is not that it is arbitrary, spiteful, and—by any standard of taxation efficiency—unsound. The worst thing about this monstrosity is its grounding in recyclable deception.Democrats on the Senate Finance Committee this month proposed the measure as a revenue offset in legislation to help small businesses. They would disallow use by the five largest oil companies of a deduction available to all US producers of physical goods since 2005 under Section 199 of the Internal Revenue Code. The deduction rate increased this year to 9% of net income from 6%—except...