I missed this post by former Republican budget staffer Keith Hennessey when it came out a couple weeks ago, but it's good enough to highlight now. Hennessey addresses the conservative complaint that nearly half of all Americans have no income tax liability. (This is not the same as paying no taxes, or even no federal taxes.) Hennessey points out that Republican policies are a major cause of this:

The reason so many Americans don’t owe income taxes is because we have two big tax credits in the code:  the Earned Income Tax Credit (EITC) and the child tax credit.  I hope the above explanation shows the power of a tax credit:  one dollar of tax credit wipes out one dollar of tax liability.  So if you provide a big tax credit to someone who owes only a small amount of income taxes, you’re probably going to move them into the non-payer category.
The EITC benefits low-wage earners.  Legislative support often splits roughly along party lines, with most Democrats wanting a bigger EITC, and many Republicans wanting a smaller (or, at least, no bigger) EITC.  Republicans like to complain about the EITC on a day like today.
But most of the increase since the mid-1990s in the number of people who owe no income taxes is the result of the child tax credit.  This policy was created by Congressional Republicans and expanded with Republicans in the lead.

Why did Republicans support the expansion of the child tax credit? One reason is pressure from Democrats who demanded less regressive tax cuts. Another is pressure from conservative Christians, who demanded that the tax code did more to encourage families.

Hennessey also notes, in passing, that the 2009 stimulus included a major tax cut that pushed even more workers below the zero income tax threshold. But it's an important point -- the statistic about "nearly half" of Americans paying no income tax is a temporary phenomenon, though I suspect many conservatives will continue to cite it after the stimulus ends and it ceases to be true.