Republicans say they won’t raise taxes. Democrats are reluctant to cut Social Security, Medicare and Medicaid. So discretionary spending — the roughly 35 percent of government that includes other social programs and the military — will have to be a big part of any deal in coming weeks to raise the debt ceiling.

But there is a little problem with discretionary spending.

According to the government’s official forecasts, discretionary spending is already slated to shrink significantly. Military spending will fall by 25 percent, as a share of the economy, over the next decade. Domestic programs will shrink even more, and by 2021 they will account for their smallest share of the economy since the 1950s.

I’m guessing you haven’t heard of these plans, however. That’s probably because plans is a bit of an exaggeration. Assumptions is a better word: per Congress’s orders, the baseline budget numbers unrealistically assume that future discretionary spending will grow only with inflation, rather than with population growth and economic growth, too.

As a result, Vice President Joe Biden, Republican leaders and the other deficit negotiators not only have to cut discretionary spending to make progress. They have to cut it even more than the Congressional Budget Office, the keeper of the official numbers, already assumes that spending will be cut.