Growth is looking good now, but it won't be enough to save the U.S. from its out-of-control budget deficit, one chief economist warned, joining a growing number of market observers concerned about government spending and debt.

"An honest accounting finds U.S. debt headed to shockingly high levels," Carl Tannenbaum, chief economist at Chicago-based asset management firm Northern Trust, said in a weekly note to clients. Tannenbaum formerly a led risk specialist division at the Federal Reserve Bank Of Chicago.

While the Tax Cuts and Jobs Act passed by Republican legislators in December 2017 will boost growth in the immediate term, along with raised caps on government spending, the fiscal stimuli come at a questionable time, he said. It has stimulated an already well-performing economy and "changed the nation's fiscal course in a potentially dangerous way," he added.

Proponents of the tax cuts predict a 3 percent growth rate to support their sunny forecasts. But the nonpartisan Congressional Budget Office (CBO) forecasts strong growth through 2018, followed by a tapering. In the long run, Tannenbaum said, "the U.S. will grow at its potential rate of 1.9 percent on average, an insufficient rate to provide enough government revenue to control the deficit."

The CBO projects the federal deficit will top a staggering $1 trillion in the next two years, and debt could equal gross domestic product (GDP) within a decade — a level not seen since World War Two. And government debt as a fraction of GDP in most developed countries is twice the level it was before the crisis, leaving little room to buffer if the world hits another economic soft patch.