GOVERNMENT WASTE is not a myth. Consider the overlap between President Trump’s newly announced plan for federal spending and President Barack Obama’s last proposed budget. Mr. Obama would have, as Mr. Trump would, eliminated several nice-sounding but dispensable programs, such as the Justice Department’s State Criminal Alien Assistance Program, Agriculture’s Water and Wastewater and Community Facilities Loan Guarantees and the Environmental Protection Agency’s Targeted Air Shed Grants.

If two of the least ideologically similar presidents in history could agree on cutting these items, they should probably be cut — if only to establish the principle that spending is not on autopilot and that the federal government does indeed care about even the minor details of its vast budget, which totaled $3.9 trillion in 2016. Such care for taxpayer funds also could have a positive effect on employee recruitment and morale.

What is a myth, however, is that waste is the fundamental cause of excessive federal debt and deficits. The three programs mentioned above cost less than $1 billion combined. Yet Mr. Trump has repeatedly fostered that myth, whether by invoking “management” as a deficit cure during the campaign or by his irresponsible promise to deal with the fiscal future while simultaneously boosting defense spending, massively cutting taxes and protecting Social Security and Medicare. True, you could have eliminated the federal deficit in 2016 by cutting all $600 billion in non-defense discretionary spending — but that would have meant closing the national parks, shutting the federal courts and putting the Coast Guard in dry dock.

The plan released by the White House on Thursday did not even claim to address taxes and entitlement programs; it focused exclusively on the $1.1 trillion discretionary budget for fiscal 2018, both non-defense and defense. This emphasis only underscores the folly of keeping revenues and mandatory spending programs out of the discussion, given the need to raise the former and trim the latter. Shifting $54 billion from non-defense discretionary spending to defense, as Mr. Trump’s plan would do, required big cuts to vital accounts. The State Department would lose almost 29 percent of its budget, and EPA, 31 percent; the National Institutes of Health would lose close to 20 percent.

As members of both parties on Capitol Hill quickly pointed out, Congress actually writes the budget, so Mr. Trump’s statement is relevant mainly for what it indicates about the executive branch’s policy priorities. In that sense, it is remarkably, if unsurprisingly, unrealistic and, as regards the United States’ posture in the world, shortsighted. “America First” reads the title of this document, which calls for spending $2.6 billion on securing the U.S.-Mexico border, including Mr. Trump’s wall. Some of that money would be taken from programs to help people abroad.

Once the uproar over Mr. Trump’s misguided, unworkable and essentially demagogic proposal dies down, Congress will have to take up the work of hammering out a real fiscal policy.