When a billionaire is mad at you, figure he can afford to stay mad, publicly, for a long time. Steve “Flat Tax” Forbes is mad at the Republican Party for a lot of things — such as its refusal to support him in the last campaign — but mostly he’s mad about his favorite topic, taxes.

Four years of traveling around the country explaining the wonders of the flat tax hasn’t given Mr. Forbes much in the way of voter support. It has allowed him to vent about the injustices of the tax system, however, as he did last week in a four-page draft of a memo he sent to a few influential party friends and, circuitously, the Washington Post. The memo is largely an exhortation to return to GOP themes. “Rose Garden photo-ops are no substitute for policies based on sound, conservative principals,” he wrote.

Such as ridding the nation of the “marriage penalty.” This tax must go, Mr. Forbes chastised, although Republicans seem to know this already, having submitted bills in both the House and Senate last fall that would try to eliminate it. But don’t try to stop a billionaire when he is on a roll.

The marriage penalty is the unintended consequences of tax policy in which two spouses with near equal income end up paying more than they would as two singles. Couples with widely disparate incomes do not have this problem, getting something of a marriage bonus. The penalty has become far more noticeable in the last decade, with more women in the workforce and the slow movement toward wage equity between the sexes.

Almost no one thinks that taxpayers should be penalized for marriage. The problem comes from trying to remove the penalty without affecting other parts of the tax system. Tax experts say it is impossible to leave the tax system progressive while also taxing families the same no matter how their income is split between spouses and eliminating the marriage penalty. The nation, in fact, has alternated between favoring singles and favoring married couples for the last half-century. The marriage penalty existed before 1948 until public grumbling got Congress to change the tax code to average the total income between spouses, helping married folks. In 1969, Congress reduced some of the tax disadvantages singles faced, creating anew the marriage penalty.

The Republican plan would adjust brackets, restore a tax deduction for the lower-paid spouse, allow couples to decide whether to file singly or jointly — and cost an estimated $20 billion. The changes erode progressivity — the idea that people with higher incomes can afford to pay a higher rate of taxes. Like the flat tax, elimination of the marriage penalty disproportionately helps the well off.

Mr. Forbes’ flat tax has not caught on and, given the details and the cost of the marriage tax break, it may not go anywhere either. But, who knows? Given his resources and angry memos, he might make the marriage tax the centerpiece of the next campaign.