During his campaign, Donald J. Trump embraced the cause of fiscal responsibility and accused President Barack Obama of shackling the country with a “mountain of debt.”

Mr. Obama “doubled our national debt. Doubled it,” Mr. Trump claimed in a speech in Virginia Beach.

Then on Wednesday, Mr. Trump unveiled the outlines of his much-anticipated tax overhaul, calling for steep tax cuts with only modest offsetting revenue increases. Economists I spoke to this week estimate it would add trillions to the national debt over the next decade.

“We’ve only done the rough numbers, but this looks like a tax cut of a magnitude of about $5 trillion,” said Maya MacGuineas, president of the Committee for a Responsible Federal Budget, a bipartisan advocacy organization for fiscal responsibility. “That is simply unimaginable given our fiscal situation and the size of the deficit, which is already the worst since World War II.”