The two parties disagree even when they agree, as they do about this: Federal spending is on an unsustainable trajectory under current law. They also agree that altering the most important drivers of this trajectory — Social Security and Medicare — is for tomorrow, which is always a day away. The parties propose, solemnly but implausibly, radically different solutions.

Republicans propose cutting taxes and regulations enough to ignite economic growth so rapid and constant that a gusher of revenue will restore fiscal health. This approach is marginally less implausible than the Democrats’ proposal, because one can at least postulate a sufficient growth rate — say, 5 percent, forever.

The Democrats’ proposal is even less realistic: “Tax the rich” until they pay their “fair share.” The Republican approach ignores political and economic probabilities. The Democratic approach ignores arithmetic.

The Congressional Budget Office’s much-too-optimistic projection is that within a decade annual budget deficits will approach $3 trillion (7.3 percent of gross domestic product.) This assumes, as the CBO is required to do, wildly improbable things: that tax cuts set to expire soon will be allowed to, that there will be no new spending initiatives and that low interest rates will minimize debt service costs.

President Biden, embracing populism’s playbook, says the burden of paying for increased government spending should be borne entirely by unpopular entities, corporations, and an unpopular minority, the rich, understood as the 2 percent of the population with annual incomes of more than $400,000. Riedl, who does not argue that high-earners’ taxes should not be raised at all.

At some tax rate below 100 percent — somewhere between 50 percent and 70 percent; say, 60 percent — revenue peaks, and higher rates would reduce revenue and increase costs to the economy. These “spillover effects” include reduced work, investing and entrepreneurship. The current top marginal tax rate, counting state and local taxes, is about 50 percent, leaving little room for aggressive income taxation to raise substantial revenue from the wealthy.

Biden’s proposed increases in capital gains taxation would produce only 0.1 percent of GDP. Just restoring the pre-2017 corporate tax rate of 35 percent (nearly 40 percent counting state taxes) would, Riedl says, “vastly exceed the rates of competing nations.”

Taxing high-earners and corporations at revenue-maximizing levels conjecturally might raise at most $7 trillion over a decade, but, Riedl warns, probably significantly less. This is because taxes on those entities and individuals would reduce incomes, wages and economic growth. The moral of Riedl’s story is: “While the highest marginal tax rate has steeply fallen over the past 80 years, federal income-tax revenues have risen as a share of the economy.” And nearly $1 trillion could be saved in a decade by curtailing subsidies to agribusinesses and trimming Social Security and Medicare benefits for wealthy retirees. The two parties could sheath their daggers and achieve progressive redistribution goals without the economic harms of large tax-rate increases.

Something they might consider — tomorrow.