Republican presidential nominee Donald Trump speaks in Wilmington, Ohio, on Friday. (Carlo Allegri/Reuters)

DONALD TRUMP has staked his presidential campaign on a nightmarish assessment of the U.S. economy. Friday’s jobs report from the Labor Department, the last before Election Day, confirms once again that his view is wrong. Unemployment fell to 4.9 percent, as employers expanded payroll by 161,000 workers. Average earnings, meanwhile, are up 2.8 percent over the past 12 months, as Americans begin to reap the benefits of a tighter labor market. Economic growth, though not rapid, is respectable — 2.9 percent in the third quarter of 2016; it has been positive in all but two quarters for the past seven years.

To be sure, there is no cause for complacency. With the epic cyclical downturn of 2008-2009 now safely behind it, the United States faces multiple long-term challenges to growth, including an inefficient tax system, sluggish productivity growth and lagging labor force participation of prime-age males. These are complex, structural issues, about which even seasoned economists possess limited knowledge — and which therefore call for measured, steady stewardship from the next president and Congress.

The prospect that voters might assign a share of that task to Mr. Trump may itself be the greatest threat to the economy. Make no mistake: Mr. Trump’s election would be a major new source of instability here and abroad, compared with which the Brexit vote would look like a hiccup. Suddenly, a man with a deep-seated hostility to, and incomprehension of, markets would be at the helm of the world’s preeminent market economy.

This may seem an odd assertion given that Mr. Trump is a businessman and that he and his advisers promise a boom based on slashing taxes (mainly for corporations and upper-income households) and deregulation. Yet even on the questionable assumption that those measures would add to growth, and not just the federal debt, their positive impact would likely be offset by policies rooted in the zero-sum instincts that Mr. Trump has voiced repeatedly in the campaign. He has promised hostility to and possible renegotiation of existing trade agreements; threatened to punish U.S. companies that invest abroad; spoken with alarming looseness about renegotiating U.S. debt; and accused Federal Reserve Chair Janet L. Yellen of manipulating interest rates for political reasons. For good measure, he has speciously accused government statisticians of fudging key economic data.

All of this portends four years of investment-chilling conflict between a mercurial chief executive and those in the business community, or in historically apolitical government economic institutions, who may displease him — or whom he chooses to scapegoat. Indeed, scapegoating is the skill this supposed business genius has demonstrated most effectively as he has pursued votes among those who still understandably feel left out of the recovery or left behind by globalization.

In blaming America’s economic difficulties on stupid trade negotiators, or immigrants, or perfidious Chinese, he has failed to level with the public about the challenges before us, to provide a plausible sense of how he would improve things — or to demonstrate much of anything except the risks of entrusting him with management of an $18 trillion economy.