WASHINGTON—Democratic presidential candidate Elizabeth Warren has unveiled sweeping tax proposals that would push federal tax rates on some billionaires and multimillionaires above 100%.

That prospect raises questions for taxpayers and the broader economy that experts are starting to ponder: Under which circumstances would taxpayers have to pay those rates? How might that change their behavior? And would investment and economic growth suffer?

Potential tax rates over 100% could result from the combination of tax increases the Massachusetts senator proposes for the very top tier of investors. She wants to return the top income-tax rate to 39.6% from 37%, impose a new 14.8% tax for Social Security, add an annual tax of up to 6% on accumulated wealth and require rich investors to pay capital-gains taxes at the same rates as other income even if they don’t sell their assets.

Consider a billionaire with a $1,000 investment who earns a 6% return, or $60, received as a capital gain, dividend or interest. If all of Ms. Warren’s taxes are implemented, he could owe 58.2% of that, or $35 in federal tax. Plus, his entire investment would incur a 6% wealth tax, i.e., at least $60. The result: taxes as high as $95 on income of $60 for a combined tax rate of 158%.

The rate would vary according to the investor’s circumstances, any state taxes, the profitability of his investments and as-yet-unspecified policy details, but tax rates of over 100% on investment income would be typical, especially for billionaires.