Tax expenditures are all the spending done through the tax code. So the deduction employers get for purchasing your health insurance, the deduction you get on the interest you pay on your mortgage, the break you get for weatherizing your house, the credit you get if you take home a very low paycheck. Because these are lost revenues rather than spending, they don’t get tallied up in the way Medicare or the military does, so you never see them in those pie charts of where the government spends its money. But as this graph from the Center on Budget and Policy Priorities shows, when you do tally them up, they’re absolutely massive:

Tax expenditures and major mandatory programs in 2010

Politicians like tax expenditures because it allows them to propose a “tax break” rather than “new spending.” The effect on the deficit is about the same, but the politics are better. The problem is that the programs often don’t work very well and they rarely get reviewed. So as Bob Greenstein says in the testimony that goes alongside that graph, they’re one of the places where deficit reduction should start. You would think there’d be some bipartisan agreement on that point, but you’d be wrong: The anti-tax pledge that most Republicans in Congress have signed says that they will “oppose any net reduction or elimination of deductions and credits, unless matched dollar for dollar by further reducing tax rates.”