In 1964 Congress passed, and Lyndon Johnson signed into law, a big income-tax cut that had originally been proposed by John F. Kennedy. The top marginal rate dropped from 91 percent to 70 percent, which represented a 23 percent cut. The bottom rate dropped from 20 percent to 14 percent, which represented a 30 percent cut. The middle rates dropped from 59 and 62 percent to 45 percent, which represented a cut of 24 percent to 27 percent. So we start out with one significant difference between the Kennedy and the Romney plan. Romney and Ryan want to lower taxes by the same amount for all brackets, high and low. Kennedy (really, Johnson) lowered taxes more at the middle and especially at the bottom than he did at the top.

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The 1964 tax cut has been embraced by supply-siders as a supply-side cut, and JFK even sold it that way to big business in a 1962 speech that supply-siders love to quote. But in fact Kennedy, and his chief economist Walter Heller, saw the tax cut as a demand-side cut aimed at creating old-fashioned Keynesian stimulus in a sluggish economy. Indeed, what Kennedy really wanted was to stimulate the economy through government spending, but he didn’t have the votes in Congress for that. So he went with the tax cut instead. The giveaway that Kennedy's wasn’t really a supply-side tax cut was that the cuts were greater in the middle and at the bottom than at the top. If you want to stimulate consumer purchasing, you’re better off concentrating income-tax cuts in the middle and at the bottom. If you want to stimulate investment, you’re better off concentrating income-tax cuts at the top—or, if that’s politically impossible, you make the cuts the same across the board. Ronald Reagan’s tax cut in 1981 was pretty obviously a supply-side cut because it lowered the top tax rate more than it did rates at the middle or the bottom. After it passed, White House budget chief David Stockman got in a lot of trouble for admitting what was obvious to anyone paying the slightest attention: The only cuts Reagan cared about were those at the top. "It's kind of hard to sell 'trickle down,'" Stockman blurted out to William Greider in the Atlantic, "so the supply-side formula was the only way to get a tax policy that was really 'trickle down.' Supply-side is 'trickle-down' theory."

If Ryan means to suggest that the 1964 tax cut paid for itself, that’s definitely not true. Despite what many supply-side fantasists maintain, tax cuts always deprive the Treasury of revenues, and the ’64 cuts were no exception. Whether the Kennedy-Johnson tax cut achieved its desired economic stimulus is a matter of some dispute. Bruce Bartlett was, in 1981, staff economist to Rep. Jack Kemp, R-N.Y., an architect of the big Reagan tax cut (and also Ryan's mentor). Bartlett gave the 1964 cut close study at the time. He says it recouped about one-third of its revenue loss through economic stimulus. But according to a recent Congressional Research Service survey, there’s no clear evidence that tax cuts have ever stimulated economic growth, going all the way back to 1945.