Look carefully at that graph. It makes two things very clear.
1) Income growth is overall much better under Democratic presidents than Republicans.
2) While the super-rich did about the same under Democrats and Republicans, the Democrats raised everybody else at the same rate while the Republicans failed everybody in the bottom 80 or 90%.
So not only do the Republicans only help the richest of the rich, but they can't even do that better than the Democrats!
The graph is from a paper (.pdf) by Princeton professor Larry Bartels. Here are some excerpts:
Census Bureau data reveal large, consistent differences in patterns of real pre-tax income growth under Democratic and Republican presidents in the post-war U.S. Democratic presidents have produced slightly more income growth for poor families than for rich families, resulting in a modest decrease in overall inequality. Republican presidents have produced a great deal more income growth for rich families than for poor families, resulting in a substantial increase in inequality. On average, families at the 95th percentile of the income distribution have experienced identical income growth under Democratic and Republican presidents, while those at the 20th percentile have experienced more than four times as much income growth under Democrats as they have under Republicans. These differences are attributable to partisan differences in unemployment (which has been 30 percent lower under Democratic presidents, on average) and GDP growth (which has been 30 percent higher under Democratic presidents, on average); both unemployment and GDP growth have much stronger effects on income growth at the bottom of the income distribution than at the top.
Under Democratic administrations income growth has been more vigorous among the poor than among the rich; under Republican administrations the reverse has been true. The cumulative effect of these differences has been enormous.
Using data from 1948 through 1978 (that is, before most of the recent substantial increase in income inequality) Hibbs (1987, 232-243) found that the ratio of the share of post-tax income received by the top 20 percent of the income distribution to the share received by the bottom 40 percent declined by about .037 during each year of Democratic control while increasing by about .008 during each year of Republican control. Applying these estimates to his entire period, Hibbs concluded that inequality declined markedly (by a total of about 25 percent) during the 14 years of Democratic control while remaining essentially unchanged during the 17 years of Republican control.
And lest you think this is just some strange coincidence:
[D]istinguishing between Democratic and Republican administrations (the black diamonds and white circles in the figure, respectively) reveals the regularity with which Democratic presidents reduced and Republican presidents increased the prevailing level of economic inequality, irrespective of the long-term trend. Indeed, the effect of presidential partisanship on income inequality turns out to be remarkably consistent throughout the second half of the 20th century. The 80/20 income ratio increased under each of the five Republican presidents in this period – Eisenhower, Nixon, Ford, Reagan, and the elder Bush. On the other hand, four of five Democratic presidents – all except Jimmy Carter – presided over declines in income inequality.