For those of us for whom this country was founded in the "New Birth of Freedom" that ended slavery and completed the Constitution with the post-Civil War Amendments, the United States WAS founded on progressive taxation.

In fact, the first progressive income tax was in 1862 to fund the Union troops. From the National Archives and Records Administration (NARA):

It was a "progressive" tax in that it initially levied a tax of 3 percent on annual incomes over $600 but less than $10,000 and a tax of 5 percent on any income over $10,000. In 1864 the rates increased and the ceiling dropped so that incomes between $600 and $5,000 were taxed at 5 percent, with a 10 percent rate on the excess over $5,000. Passed as an emergency measure to finance the Union cause in the Civil War, the first income tax generated approximately $55 million in government revenues during the war. Paying the taxes was viewed as part of the patriotic war effort, and the whole country was proud when the merchant prince A. T. Stewart paid $400,000 in taxes on an income of $4 million.

It's worth noting that the Congress assumed it had the power to pass an income tax on that basis and revived it in the 1890s, only to have a rightwing Supreme Court strike the income tax down. This led to passage of the 16th Amendment, further ratifying progressive taxation as a founding principle of our constitutional system.

One thing that conservatives don't understand about America-- liberty was not founded in 1776. It's been a continual process with each generation having to refound this country in struggle -- a little bit of revolution periodically in Thomas Jefferson's phrase. But there is little question that our modern constitutional system was only established with the Civil War, and progressive taxation was at the heart of that effort.