The Buffett rule is named after Warren Buffett, who believes it is unfair that his secretary, who makes far less than he does, pays tax at a higher marginal rate. One version, the Paying a Fair Share Act of 2012, is scheduled by Senate Democrats for a procedural vote on April 16th—as it happens, a day before federal income-tax returns must be filed. The rule aims to ensure that people earning more than $1m a year pay tax at an effective rate of 30%.

Many do not. A report issued by the White House a day ahead of Mr Obama’s speech found that in 2009, 22,000 households earning more than $1m paid less than 15% of their income in taxes, and 1,470 households paidno federal income tax at all. Since around 1980, average tax rates of the top 1% of American earners have fallen as their incomes have risen. Small wonder that the president proposed the Buffett rule, as Jason Furman, deputy director of the National Economic Council put it, “as a basic principle of tax fairness”.

Alan Viard, an economist at the American Enterprise Institute, a conservative think-tank, describes the bill far less sexily: “It boils down to a partial rollback of the preferential rate for long-term capital gains.” Millionaires will retain their deductions for charitable donations, and those who already pay more than 30%—earning their millions in a payroll job, say, rather than through tax-advantaged investment—will generally be unaffected.

Paul Ryan, a Republican congressman from Wisconsin who is emerging as Mr Obama’s foil on economic matters, complained that “people think the Buffett rule is sort of budget pixie-dust” that is “going to fix our fiscal problems”, when in fact it is forecast to raise around $47 billion in ten years, according to congressional estimates: a drop in the proverbial bucket compared with both current deficit levels and projected spending levels over that period. Republicans are also quick to point out that the question of fairness cuts both ways. The top 1% of earners already pay 40% of federal income taxes, while nearly half of all Americans pay no federal income taxes at all. Orrin Hatch, the ranking Republican on the Senate Finance Committee, dinged the bill for “being designed for no other reason than politics.”

But politics is now the point. With Rick Santorum’s exit from the Republican race, the general-election season has arrived. Scott Hodge, who heads the right-leaning Tax Foundation, calls the Buffett rule “a bludgeon to hit Mitt Romney with”. Indeed it is. When Mr Obama talks about millionaires using accountants, lawyers and tax-advantaged investments to lower their tax rates, he is talking about Mr Romney—whose net worth is around $200m, and who in the past two years made $42.6m, mostly from capital gains, and paid around 15% of his income in taxes.