Congress has been slugging it out for years over ways to cap spending. But one huge cause of the budget deficit is often ignored: tax loopholes. According to a new General Accounting Office report, special-interest tax breaks will cost the Treasury $400 billion in revenue this year--twice the deficit. And while spending for defense and many domestic programs is frozen, tax breaks keep growing at a 4% annual clip, after inflation. Now, the GAO and the Office of Management & Budget hope to curb the revenue drain by turning a spotlight on the loopholes. And House Ways & Means Committee member William J. Coyne (D-Pa.) is pushing the panel to review the breaks as a way to reduce the deficit.