Warren Buffett and Jamie Dimon want billionaires to pay their fair share to Uncle Sam. But a ‘millionaires tax’ would likely affect all the wrong people

America’s most celebrated investor and the head of Wall Street’s most powerful bank have had a clear message for Washington: Tax us more.  Warren Buffett and Jamie Dimon, the CEOs of Berkshire Hathaway and JP Morgan Chase, respectively, have both advocated for raising taxes on the rich as a matter of fairness—and to address a burgeoning federal deficit.

Congressional Republicans, traditionally opposed to hiking taxes on the wealthy, are reportedly mulling a so-called “millionaires tax,”...