The desire of many lawmakers to rein in such supplemental appropriations and reassert meaningful control over all annually appropriated funds — and the practice the Obama Administration has followed of including war funding within the regular defense appropriations bill, which has improved budget transparency — would become much harder to fulfill if biennial budgeting were implemented. It is not possible for Congress effectively to plan ahead for unexpected needs in the second year of a biennium. Large supplemental appropriations to meet such needs outside of the two-year budget plan would almost certainly become a regular part of the budget process and could further erode budget controls and accountability.
(Note: A recent paper from Cato adjunct scholar Veronique de Rugy explains that supplemental appropriations are already a problem.)
As a former budget official in a state that uses biennial budgeting, I just don’t understand what congressional Republicans think they’re going to accomplish. The cynic in me thinks that at least part of the support stems from the unwillingness of most Republicans to get specific on what they’d eliminate from the federal budget. Like the Balanced Budget Amendment, I think a lot of Republicans are simply using biennial budgeting as political cover.