When asked last May whether the U.S. needed to fully repay its public debt, Donald Trump said, "I would borrow, knowing that if the economy crashed, you could make a deal."

Such remarks, by a modern presidential candidate, are unprecedented. The U.S. is able to borrow money around the world at the lowest interest rates because Treasury securities are regarded as absolutely safe.

If investor confidence is shaken by irresponsible comments, it could cost American taxpayers an enormous amount of money, for a long time.

Creditors everywhere would demand much higher interest rates on U.S. bonds. That would immediately increase the amount of interest the U.S. would have to pay on the money it borrows.

HIGHER INTEREST RATES

The large, but currently manageable, debt would become unmanageable with a major increase in interest rates. The U.S. would move rapidly toward a government debt crisis.

At the same time, the value of U.S. Treasuries would collapse on world markets. Since Treasury bonds are part of investment portfolios all over the world, literally everyone would take a hit.

Apparently Trump was either unaware or never bothered to think it through.

His tax plan would significantly reduce marginal tax rates on individuals and businesses, increase standard deduction amounts to nearly four times current levels, and curtail many tax expenditures (deductions, credits allowances and deferrals - a good thing).

His proposal would cut taxes at all income levels, although the largest benefits, in dollar and percentage terms, would go to the highest-income households.

REVENUES WOULD BE REDUCED

Ultimately, the plan would reduce federal revenues by $9.5 trillion over its first decade before accounting for added interest costs, according to the non-partisan Tax Foundation, the nation's leading independent tax policy research organization.

Unless Trump is able to implement very large spending cuts, his suggested tax plan would result in a huge increase in the national debt; an increase of about $11 trillion - on top of the large increase in the federal debt already projected, by the Congressional Budget Office, under current law; assuming Congress does nothing new.

HUGE INCREASE IN DEBT

This would result in a total national debt in 2027 of about 144 percent of gross domestic product vs. 102 percent currently. The country would be in a debt crisis well before then.

Unfortunately, Trump has not offered a plan to restrain the overall growth in government spending. It will be extremely difficult to have a sustainable tax cut of any significant size unless those spending limits are in place. In reality, outside of his vague rhetoric about "waste, fraud and abuse," Trump has proposed nothing on the spending side.

FOCUS ON ENTITLEMENTS

The real money in the federal budget can be found in entitlements (Social Security, Medicare, Medicaid, Obamacare and a few others) and defense.

These represent America's biggest long-run fiscal challenge, especially because of an aging population. Yet Trump says he wants to leave most of these programs untouched (except Obamacare).

It doesn't compute.

Trump has made a number of statements in the economics sphere and offered economic proposals that might be just fine in the business world but are either wrong or impractical for the government.

The halls of government at all levels are littered with successful businessmen who did not understand that business acumen doesn't necessarily translate into political effectiveness.

Trump is just the latest example.

� Joseph B. Steinman is a retired finance professor.

� He lives in Ponte Vedra Beach.