Back here I asked the question whether an unincorporated 527 association can accept unlimited contributions for electioneering communications. It appeared that the answer was yes, so long as the 527 was not incorporated, took no corporate or union funds, and did not engage in express advocacy.
It turns out that there is an argument to limit 527 contributions—that such organizations are “major purpose” organizations that can be regulated as political committees under the FECA, the same way that political parties are regulated. For an argument in favor of this interpretation, see Donald Tobin, Election Speech and Section 527 of the Internal Revenue Code, 37 Ga. Law Review 611, 688-691 (2003). I have not yet looked into this issue myself and would be interested in hearing counter-arguments.