GOP bill could add trillions to the U.S. deficit, some experts say. Here's what to know.

The Republican-backed tax and spending bill that's now moving forward in the House is getting a reality check from Wall Street. That's because the proposed tax breaks are projected to far outstrip any savings in the bill, potentially leading to mounting U.S. debt and a worsening fiscal outlook, according to economists and policy experts. 

If the bill moves forward without significant changes, the U.S. would likely find its debt levels spiraling higher in order to finance the tax cuts, according to several analyses. Over the next decade, the GOP bill could cost the U.S. $3.8 trillion, according to a report earlier this month by the Joint Committee on Taxation that examined the impact of the the tax measures versus spending cuts. 

Those risks are rattling Wall Street. Moody's Ratings on Friday downgraded the nation's debt, citing the potential for the new bill to add $4 trillion to the federal primary deficit, which excludes interest payments, over the next decade.