Donald Trump’s re-election campaign will be centered around claims that he has done great things for the economy. And let’s be honest: The U.S. economy is running pretty hot these days. Growth in G.D.P. and employment has been good, though not spectacular; the unemployment rate is near a historic low.

There are some shadows in the picture. Economic gains have been lopsided, with a large increase in corporate profits that mainly reflects giant tax breaks, while workers haven’t seen comparable gains (and gains for lower-wage workers have been driven in part by minimum-wage increases in blue states). The huge gains in health insurance coverage under President Barack Obama have stalled or gone into reverse, and there has been a sharp increase in the number of Americans who report delaying medical treatment because of costs.

Still, it is indeed a strong economy. But if we ask what lies behind that strength, the main answer is an explosion in the federal budget deficit, which exceeded $1 trillion last year. And the story of how that happened has deeply disturbing implications for the future of U.S. politics.

Let’s go back for a minute to early 2009, when the economy was imploding and desperately needed help in the form of deficit spending. The Obama administration did indeed propose a significant stimulus plan — but it was much too small relative to the size of the problem, largely because the administration wanted to win bipartisan support and wasn’t willing to use reconciliation to bypass the filibuster.