A taxpayer who thinks that $600 billion is too much to spend on military in the post-Cold War era could choose to allocate less to that function than the government requested. A taxpayer who thinks that Congress has been underfunding Head Start and the arts could allocate double the requested amount for those programs.
There would be quite a bit of debate, of course, over how to list programs in the 1040-D program. Spending interests would want to use broad categories–national defense, health, education, job training. Opponents of spending would prefer to narrow the categories so taxpayers can see what they’re really buying– defense of Japan and Korea, war in Iraq, farm subsidies, mass-transit “demonstration” projects in West Virginia, and so on. Libertarians and the arts establishment might agree on listing just “arts,” while the religious right might lobby to have the category broken into “fine arts,” “pork-barrel arts,” and “obscene art.” Language would be an issue – “corporate welfare” or “loans for small businesses”?