Fitch Ratings, one of the three major firms assessing the U.S. government's creditworthiness, says President Trump's tax cuts are highly unlikely to pay for themselves, contradicting Treasury Secretary Steven Mnuchin's repeated assertions that they would do just that.

The Tax Cuts & Jobs Act bill, introduced last week in the U.S. House of Representatives, will lead to wider fiscal deficits and add significantly to U.S. government debt, Fitch said Tuesday in a statement. Federal debt will rise to 120% of gross domestic product in a decade from 77% currently, according to the ratings firm.

At the core of Fitch's doubt is the Trump administration's oft-repeated mantra that tax cuts for companies from General Electric Co. (GE) to Bank of America Corp. (BAC) and Apple Inc. (AAPL) will deliver a long-term fillip to economic growth by stimulating corporate investment and creating new jobs. Instead, the ratings firm expects a "modest and temporary spur to growth," with GDP rising to 2.5% in 2018 but then falling back to 2.2% in 2019, Fitch said. This year, the economy is projected to grow an estimated 2.2%, according to a survey by financial-data provider FactSet.

"Tax cuts may lead to a short-lived boost to output, but Fitch believes that they will not pay for themselves or lead to a permanently higher growth rate," the ratings firm said in the statement.

The commentary by Fitch directly counters Mnuchin's assertions that the tax cuts will spur growth, thus generating enough additional tax revenue -- as much as $2 trillion -- to pay down debt. In September, the Treasury secretary projected that the tax cuts would help push economic gains to 2.9% over 10 years.

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Fitch didn't say whether the tax cuts might endanger the U.S. government's triple-A rating but noted that the country's "debt dynamics" are worse than those of any other countries with similar ratings. The U.S. is able to keep its top-shelf rating partly by virtue of the U.S. dollar's status as the main reserve currency for central banks around the world, according to Fitch.

"The U.S. is the most-indebted AAA country, and it is running the loosest fiscal stance," Fitch said. "Long-term debt dynamics are also more negative than those of peers, with health and social-security spending commitments set to rise over the next decade."

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