President Donald Trump’s proposal to balance the budget within 10 years includes some aggressive and potentially inconsistent economic assumptions and accounting devices.

One example: The White House predicts economic growth will pick up speed in the coming years without much increase in either inflation or interest rates. That, in turn, is expected to boost revenue growth and reduce the need for social-safety-net spending. To achieve a fortuitous mix of fast growth and low inflation and interest rates, the economy would need...