Tax cuts and a slowing economy will erode America’s fiscal strength during the next decade, according to a new report from Moody’s Investor Service, the bond-rating agency. At some point, Moody’s might cut the nation’s top-tier credit rating.

The Moody’s warning challenges a core promise of President Trump and his fellow Republicans, who insisted the $1.5 trillion tax cut they passed last year would pay for itself and even generate more tax revenue, not less, because economic growth would suddenly boom. Trump predicted last year that the economy would “take off like a rocket ship” once his tax cuts went into effect. White House economists predicted family incomes would rise by $4,000 or more due to a sharp cut in business taxes.

None of that is happening or coming into view. The economy grew at a robust 4.2% in the second quarter, the highest level since 2014. But Moody’s Analytics predicts growth of just 2.9% for all of 2018, and 2019 as well. It will then fall to 0.9%, according to the forecasting firm. If so, economic growth under Trump would average just 2.2% per year, almost exactly the same as during President Obama’s second term.

The federal budget deficit, meanwhile, rose from 3.5% of GDP in 2017 to 3.8% in 2018. Moody’s expects it to hit 4.8% of GDP in the current fiscal and soar to 8% by 2028. The U.S. fiscal debt burden is the heaviest among nations that earn Moody’s Aaa rating, its highest.

When the economy’s strong, as it is now, the government’s debt burden normally declines rather than spiking. That’s because businesses and individuals earn more money and therefore pay more taxes. But the Trump tax cuts have weakened revenue intake, pushing deficits up.

Tax revenue from businesses fell by 31% in fiscal 2018, which ended in September. That decline mostly came from cutting the corporate tax rate from 35% to 20%. Tax revenue from individuals rose by 6.1% in 2018, but that was mostly because of inflation, population growth and 1.7 million new jobs created during 2018.

The deficit rose from $666 billion in 2017, before the tax cuts, to $779 billion in 2018, when the tax cuts had been in effect for eight months. The deficit is likely to approach $1 trillion this year, an unprecedented gap for an economy that’s supposedly booming. During the late 1990s and early 2000s, unemployment was roughly as low as it is now, but the government ran a budget surplus for four years in a row.

“The United States’ fiscal strength is set to gradually decline from 2019 onward,” Moody’s Analysts wrote in the report. “A persistent widening of fiscal deficits will push the federal debt and interest burdens to historic levels, which will ultimately weigh on the sovereign credit profile.”