A FAIR assessment of the nation’s fiscal predicament would be: troubling but fixable. That is the picture that emerges from the latest Congressional Budget Office budget report , released just after President Trump took office on Jan. 20. Absent changes in current law, federal debt held by the public will rise from 77 percent of national output to 89 percent by 2027 — far above post-World War II norms, and threatening the government’s ability to fund existing military and domestic needs, let alone respond to emergencies such as war or recession. The cause of the escalating debt would be a chronic excess of spending over revenue; the spending would be driven primarily by entitlement programs such as Medicare, plus interest on the debt. Discretionary spending, which includes defense, would dwindle to a mere 5.3 percent of output, the lowest level since comparable data reporting began in 1962.

This impending squeeze could be averted with a relative modicum of shared sacrifice: raise revenue through selective measures targeted at those most able to pay; trim entitlement spending; right-size or eliminate inefficient programs.

As President Trump prepared to address Congress Tuesday on his policy agenda, however, it was becoming distressingly likely that the plans he has would make the fiscal situation far worse. Treasury Secretary Steven Mnuchin said Sunday that the president’s first budget, due March 13, would not touch entitlements, but would cut taxes for business and the middle class. On Monday, White House officials told reporters to expect Mr. Trump to call for $54 billion in additional defense spending to shore up the admittedly stressed military. The president followed that up with an impromptu lecture on the crumbling ceiling tiles of tunnels in New York City, a reminder that he has also pledged to support a major increase in spending on infrastructure.

As for paying for all of this, the closest thing to a specific proposal was the notion, floated by the White House, of offsetting the big defense hike by cutting foreign aid, the Environmental Protection Agency and other discretionary programs. Reality check: The combined budget for the EPA and the State Department was only about $46 billion in the current fiscal year. Even eliminating them entirely could not pay for the defense boost Mr. Trump is apparently contemplating.

To govern is to choose. To govern populistically is to promise favored constituencies — in Mr. Trump’s case, the military, corporations and construction unions — the moon, then pass the tab to future generations. In country after country around the world, this has been a formula for short-term political gain — and long-term economic trouble. The only thing standing between the United States and a similar spiral may be Congress, dominated by Mr. Trump’s uneasy Republican allies.

Will the GOP’s leaders on Capitol Hill stand up for the principles of fiscal responsibility they so loudly proclaimed when a Democrat, Barack Obama, occupied the White House? Or will they capitulate to Mr. Trump on this issue, as they have on so many others before?