Increasing tax rates doesn't lead to a proportionate increase in tax revenue, because tax rates have a dynamic effect on economic activity, and because high-income people can find legal ways to shelter income from taxation. I'm not saying that there hasn't been too much political solicitude to the donor class (and not just among the GOP), but "just raise taxes" is not an easy solution to every fiscal problem. "Just raise taxes on the rich" is what many Dems believe will pay for some massive program whose cost exceeds the total wealth of the super-rich class they're targeting. They never want to consider a reduction of government spending -- except on defense.