When Steve Forbes ran for president in 1996 on a plan that called for no taxes on dividends and capital gains, Mitt Romney, then a private citizen, took out a full-page ad in The Boston Globe attacking his proposal as plutocratic.

“The Forbes tax isn’t a flat tax at all — it’s a tax cut for fat cats!” Mr. Romney’s ad declared, noting that “Kennedys, Rockefellers and Forbes” could end up with a tax rate of zero, while ordinary people would be left paying 17 percent on their wage and salary income under Mr. Forbes’s plan.

The mainstream Republican position on capital gains has long been that they should be taxed at a low rate, but not zero. In 1996, Mr. Romney was supporting Bob Dole, the eventual nominee, whose campaign platform called for a 14 percent tax rate on capital gains. In 2003, President George W. Bush signed a law setting the rate at 15 percent, a policy that John McCain proposed to continue if elected in 2008. (The current maximum rate on capital gains is 23.8 percent, after tax increases that took effect in 2013.)

When Mr. Romney was the Republican nominee in 2012, he proposed to abolish the capital gains tax for moderate earners — who typically have few capital gains anyway — but not for the Kennedys, Rockefellers and Forbeses, who would have continued paying 15 percent.