From Taegan Goddard's Political Wire, Marco Rubio's message can basically be boiled down to a combination of "thanks for the campaign money" and "it's not me, it's radical you":

For Misuse of 501c4 Status To Hide Donors And Engage In Prohibited Political Activity

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Washington, DC — The watchdog group, www.AmericanCrossroadsWatch.org, hails Senate Finance Chair Max Baucus for requesting that the Internal Revenue Service (1) survey major 501(c)(4), (c)(5) and (c)(6) organizations involved in political campaign activity to examine whether they are operated for the organization’s intended tax exempt purpose and to ensure that political campaign activity is not the organization’s primary activity, (2) examine if these political activities reach (a primary purpose level–) the standard imposed by the federal tax code – and if they do not, whether the organization is complying with the notice or proxy tax requirements of Section 6033(e), and (3) survey major 501(c)(4), (c)(5), and (c)(6) organizations to determine whether they are acting as conduits for major donors advancing their own private interests regarding legislation or political campaigns, or are providing major donors with excess benefits. See complete letter at http://www.velvetrevolution.us/images/Baucus_irs_tax_exempt_9_28_10.pdf