Party leaders have promised not to touch Medicare and Social Security. Republicans generally oppose reductions in military spending and veterans’ benefits. And neither party can do anything about interest payments on the debt that the government has already accumulated. Combined, these categories make up almost two-thirds of federal government spending.

The largest remaining category involves health care spending that benefits lower- and middle-income families, including from Medicaid and Obamacare. Hard-right Republicans, like some in the Freedom Caucus, have signaled they will propose reductions to these programs. Party leaders, for their part, have said they would eye cuts to anti-poverty programs such as food stamps.

But cuts like these would have a big potential downside for Republicans: The partisan shifts of recent years mean that Republican voters now benefit from these redistributive programs even more than Democratic voters do.

As The Atlantic’s Ronald Brownstein recently wrote, “The escalating confrontation between the parties over the federal budget rests on a fundamental paradox: The Republican majority in the House of Representatives is now more likely than Democrats to represent districts filled with older and lower-income voters who rely on the social programs that the G.O.P. wants to cut.”

Almost 70 percent of House Republicans represent districts where the median income is lower than the national median, according to researchers at the University of Southern California. By contrast, about 60 percent of House Democrats represent districts more affluent than the median.

The politics of class, as Brownstein puts it, have been inverted.