WASHINGTON—President Trump is expected to release a $4.8 trillion budget Monday that charts a path for a potential second term, proposing steep reductions in social-safety-net programs and foreign aid and higher outlays for defense and veterans.

The plan would increase military spending 0.3%, to $740.5 billion for fiscal year 2021, which begins Oct. 1, according to a senior administration official. The proposal would lower nondefense spending by 5%, to $590 billion, below the level Congress and the president agreed to in a...