The Congressional Budget Office’s dismissal of a legislated cap on war spending marks a rare case in which the nonpartisan research arm of Congress effectively tossed out an offset as essentially mistaken accounting.

In a Feb. 5 letter responding to a request from House Budget Chairman Paul D. Ryan, R-Wis., CBO Director Douglas W. Elmendorf said the agency would not consider a legislated cap on war spending, a category of spending that is appropriated and is classified as discretionary, as an offset for increases in mandatory spending programs.

“Congressional scorekeeping procedures do not permit budgetary effects in those two categories to be combined,” he wrote, restating CBO views on the offset.

Elmendorf added that “appropriations for war-related activities have declined in recent years and may decline further as military operations in Afghanistan wind down.”

He said caps on war spending “that are lower than baseline projections might simply reflect policy decisions that have already been made and that would be realized even without such funding constraints.”

Under congressional scoring rules, the CBO is obliged to show appropriated programs, including emergency war spending, as growing at the rate of inflation in its 10-year baseline.

However, as the agency has noted, the baseline is meant to be a neutral benchmark against which lawmakers can measure the effects of proposed legislation on spending, revenue and the deficit, not a projection of budgetary outcomes.