The Tax Foundation is a libertarian organization.  Nancy MacLean, in her book, suggests Libertarian is synonymous with "Neoliberal", "Conservative" and "Republican".  In other words, it represents the interests of American Oligarchs who define "liberty" as "economic liberty" where a rich man can buy what ever he wants, including Congressmen.

Much of the financing for the Tax Foundation comes from the Koch Brothers over $1M since 2001.  To best understand who they represent, check out their web site and the people they give "distinguished service awards" to.