Sen. Al Franken (D-Minn.) speaks about net neutrality during a discussion hosted by the Free Press Action Fund on Capitol Hill on July 8. (Saul Loeb/AFP/Getty Images)

One of the most vocal skeptics of industry consolidation, Sen. Al Franken hasn't pulled any punches when it comes to the proposed merger between Comcast and Time Warner Cable — a deal that would give Comcast control over roughly 30 percent of the pay-TV market.

Now, the lawmaker is setting his sights on another major deal: AT&T's proposal to acquire DirecTV. In a letter to federal regulators, Franken warns that letting the deal go through could turn AT&T into a gatekeeper to the mobile Internet. Franken also complains that AT&T took inappropriate steps to block Internet applications like Google Voice and Skype.

"AT&T has a history of skirting the spirit, and perhaps the letter" of the government's rules on net neutrality, Franken wrote. Those rules were written in 2010 — and AT&T has pledged to abide by them for three years if its DirecTV purchase goes through — but were knocked down by a federal court in January. Read the rest of the letter below: