As you will see, in 2004 (the most recent tax year for which these statistics are available, the top 1% of taxpayers paid 36.89% of Federal Income taxes.  That is the largest % paid from 1986-2004 by this income group.  From this same factual data, you will see the same statement is true for the top 5%, top 10%, top 25% and top 50%.  The top 50% paid 96.7% of the tax bill in 2004, the highest % over the 1986--2004.  The US federal tax system has become more progressive over the last 18 years.  It's just incredible that you are ignoring the data.

A quick remark without going into too much details. From the IRS data it's clear that increase in tax bill of the first 1% is driving much of the increase for other top percentiles, just because top 1% pay so much of the total tax. Therefore, a quick and dirty test of progressivity trends in the US tax system would be compare the changes in total income share of the top 1% with the changes in their total wage bill.