I've never heard anything partisan about the The Tax Foundation or the Tax Policy Center though, and they are the basis for most of these numbers (I could be totally wrong).
The Tax Foundation is the source for a lot of bad information. This organization was founded in 1937 by Republicans to oppose Social Security.
Tax Foundation Figures Produce Misleading and Inaccurate
Impressions of Middle Class Tax Burdens
The Tax Foundation stat that is most misleading is that "43%" of American tax filers pay no income tax.
Read the misleading logic on this page:
"During 2006, Tax Foundation economists estimate 43.4 million tax returns (from) 91 million individuals face a zero or negative tax liability…out of 136 million federal tax returns. Add 15 million households and individuals who file no tax return at all, roughly 121 million Americans—or 41 percent of U.S. population—will be completely outside the federal income tax system in 2006."
See real IRS data here
Out of 92,713,707 tax returns with taxable income, 5,987,541 paid no income tax. Only 6.5% of those with taxable income paid no taxes.
Tax Foundation inflated its claims in a number of ways. One way is the Tax Foundation counts 8 people in every of the 15 million household who file no tax return.
The misinformation is real bad from conservative sources. You need to check the source material for their claims. Fortunately the Web allows yo to find primary source data to discern the truth.