Republican presidential candidate Donald Trump unveiled an ambitious tax plan Monday that he says would eliminate income taxes for millions of households, lower the tax rate on all businesses to 15% and change tax treatment of companies’ overseas earnings.

Under the Trump plan, no federal income tax would be levied against individuals earning less than $25,000 and married couples earning less than $50,000. The Trump campaign estimates that would reduce taxes to zero for 31 million households that currently pay at least some...