Republicans are noticeably silent. Whereas the anti-establishment populism of the Tea Party put pressure on Republicans to address the problem, Mr. Trump’s brand of populism has moved the party in the opposite direction. In July, when a caller to Rush Limbaugh expressed concern about the return of $1 trillion deficits under Mr. Trump, the radio host, who has always had his hand on the pulse of his audience, responded: “Nobody is a fiscal conservative anymore. All this talk about concern for the deficit and the budget has been bogus for as long as it’s been around.”

Dismissing fiscal conservatism as a bogus issue is a serious mistake that poses an existential threat to a movement that was built around limiting the burden that government places on individuals. Republicans may convince themselves that they can cut taxes every time they take power, but the reality is that if the long-term debt is not put on a manageable trajectory through reasonable reforms, it will require enormous, crushing tax increases just to sustain existing government programs.

With leading Democratic presidential candidates proposing tens of trillions of dollars of new federal spending, Republicans’ abdication of fiscal conservatism leaves Americans with no responsible party. If nothing changes, the risk of a fiscal crisis rises, and future generations face catastrophic consequences.

The shift in tone among Trump Republicans cannot be attributable to suddenly rosy forecasts. The C.B.O. warned in June that the debt was still on an “unprecedented” course. In 2019, public debt reached 78 percent of economic output. That’s higher than the debt that existed at the peak of the Civil War, World War I and the Great Depression. The only time in American history when debt exceeded current levels was during World War II. But in that case, once the fighting stopped, the debt started to retreat.

In contrast, the current debt outlook is not from a one-off event, such as a major war or economic downturn, but from the combination of rising health care costs and an increase in the retirement-age population that is driving up spending on Medicare and Social Security (the C.B.O. includes future unfunded obligations for the latter in its projections). In the coming decades, debt is projected to blow past the World War II record and reach 144 percent of gross domestic product by 2049.