President Obama accomplished two things when he made the case on Tuesday for the so-called Buffett Rule, which would require millionaires to pay at least 30 percent of their income in taxes. He persuasively argued that it would be a step toward fairness in a tax code tilted in favor of the wealthiest Americans. Not incidentally, it allowed him to take an implicit shot at his virtually certain opponent, Mitt Romney, both personally and politically.

Mr. Romney disclosed in January that his tax bill last year came to about 14 percent of his $21 million income, roughly the same percentage faced by middle-rung taxpayers. Even more important, Mr. Romney is determined to continue slashing taxes for the rich, starving the nation of needed revenue, while deepening the deficit.

The Buffett Rule, which would raise an estimated $50 billion over 10 years, would not make an appreciable dent in the deficit or provide a lot more for essential programs. By comparison, letting the Bush-era tax cuts expire for taxpayers making more than $250,000 a year, as the president has also called for, would raise $800 billion over 10 years.

Mr. Obama must ensure that the Buffett Rule does not become a substitute for ending those tax cuts.

The president is right that income inequality is a serious and growing problem and should be a central issue in this year’s campaign. On Tuesday, he said the big question for Americans is, can “we succeed as a nation where a shrinking number of people are doing really, really well, but a growing number are struggling to get by? Or are we better off when everybody gets a fair shot?”