Is he dreaming?

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 income tax. The highest individual income-tax rate would be 25%, compared with the current 39.6% rate.

Many middle-income households would have a lower tax rate under Mr. Trump’s proposal, but because high-income households generally pay income tax at much higher rates, his proposed across-the-board rate cut could have a positive impact on them, too. For example, an analysis of Jeb Bush’s plan—taxing individuals’ incomes at no more than 28%—by the business-backed Tax Foundation found that the biggest percentage winners in after-tax income would be the top 1% of earners.

Mr. Trump’s plan appears designed to help him, as the GOP front-runner, cement his standing as a populist—though that message is complicated by the fact that the billionaire, like other Republican leaders, would eliminate the estate tax.

“My plan will bring sanity, common sense and simplification to our country’s catastrophic tax code,” Mr. Trump said in an interview. “It will create jobs and incentives of all kinds while simultaneously growing the economy.”

But Mr. Trump will face a challenge in convincing skeptics that his aggressive tax cuts can be implemented without adding to the federal deficit.

To pay for the proposed tax benefits, the Trump plan would eliminate or reduce deductions and loopholes to high-income taxpayers, and would curb some deductions and other breaks for middle-class taxpayers by capping the level of individual deductions, a politically dicey proposition. Mr. Trump also would end the “carried interest” tax break, which allows many investment-fund managers to pay lower taxes on much of their compensation.

A significant revenue gain would come from a one-time tax on overseas profits that could encourage U.S. multinational corporations to return an estimated $2.1 trillion in cash now sitting offshore, largely to avoid U.S. taxes. His proposal would impose a mandatory 10% tax on all of that money, even if the money stays overseas, but allow a few years for the tax to be paid. The Trump campaign estimates that many companies would choose to bring their money back home, boosting jobs and investment in the U.S.