WASHINGTON—President Trump’s $4.8 trillion budget charts a path for a potential second term, calling for steep reductions in social safety-net programs and higher outlays for defense and the space program, while assuming tax cuts will be extended.

The plan, released on Monday, would increase defense spending 0.3%, to $740.5 billion for fiscal year 2021, which begins Oct. 1. It would lower nondefense spending by 5%, to $590 billion, below the level Congress and Mr. Trump agreed to in a two-year budget deal last summer.

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