If Congress does nothing, the cuts set to start today will stay in place and the deficit would reach 2.4 percent of GDP by 2015, according to the CBO. The deficit then would start rising, reaching 3.8 percent of GDP by 2022.

Discretionary Programs

Discretionary programs are easier to cut because their budgets are set annually rather than locked into law by benefit formulas. Lawmakers are also reluctant to impose entitlement changes immediately, preferring to protect current retirees -- who vote -- and phasing in changes over time.

“There is no question that for a long time, the long-term budget outlook has been driven by the mandatory spending, health spending in particular,” Douglas Holtz-Eakin, a former CBO director and adviser to 2008 Republican presidential candidate John McCain, told the Senate Finance Committee Feb. 26. “You’re not going to grow your way out of it. It’s been clear for a long time you can’t patch your way out of it.”

The result means less money for infrastructure, education, and defense. Deficit reduction has kept the economy below its capacity for several years, said Andrew Fieldhouse, federal budget policy analyst at the Economic Policy Institute in Washington, which favors helping low- and middle-income workers.

“Timing is everything,” Fieldhouse said. “And if you cut back now, you’re actually going to make it harder to address your long-term challenges because austerity in a depressed economy is particularly damaging.”

Diverging Approaches

Democrats say higher revenue must be part of the solution. An aging population and rising health-care costs, they say, mean that the long-run average of revenues at about 18 percent of GDP can’t be sustained.

“We want a balanced approach: we want revenues, we want spending cuts, a nice, even balance on both,” Senator Tom Harkin, an Iowa Democrat and Appropriations Committee member, said in an interview. “I think the Republicans are going to be held accountable, dearly accountable, for this.”

Republicans say a spending-only approach to deficit reduction is essential.