WASHINGTON—The Congressional Budget Office estimates the government will take in $1.9 trillion less in revenue and spend $300 billion more over the next decade than the White House estimated under its latest budget proposal if the plan were enacted.

President Donald Trump in February proposed a $4.4 trillion budget that would boost federal spending for the military and border security, cut many domestic programs and projected deficits through the next decade. The proposal depends heavily on cuts to government safety-net programs...