OK I have been indignant about usually reliable people ascribing tax cuts for the working poor in significant part to Republicans. This is true of one important tax cut -- the child tax credit which was part of the Bush signed 2001 tax cuts.
But there is widespread belief that the EITC was significantly expanded (or even created) by Republicans (OK Ford signed the bill). It was introduced in 1975 at at time when Democrats had huge majorities in both houses of Congress. It was massively expanded in 1993 by the Clinton tax bill which was supported by no (not one,zero,nessuno) Republican in Congress. Reagan and Clngress (including the Democratic majority in the house) increased the payroll tax. That is a tax which Romney tends to assert doesn't exist, but a dollar of payroll taxes is as much of a disincentive to work as a dollar of income tax.
To try to understand the changes I turn to Piketty and Saez (warning pdf) and in particular Table A3 "Average Tax Rates by Income Groups 1960-2004." I will mainly look at the sum of the individual income tax and the payroll tax. My claims
1)The sum of tax rates on low incomes increased under Reagan (the payroll tax increase outweighed income tax cuts)
2) That sum of rates was sharply cut from 1993 to 1995 (by the bill with no Republican support)
3) The average income tax rate on low incomes changed little from 1996 to 1998 -- welfare reform did not include significant tax cuts for the poor.
4) the sum declined sharply from 2001 to 2004
The table is huge and I will try to cut out comprehensible parts