It's no secret that Republicans want to replace the Federal Communications Commission's new net neutrality regulations with legislation. But they need Democrats to do it — and at least one prominent liberal is signaling that he won't go along with the plan unless the GOP substantially changes the deal on the table.

Sen. Bill Nelson (Fla.) is the top Democrat on the Senate Commerce Committee. On Wednesday, he reiterated what he's been saying for weeks: That he's open to working with Republicans on a "truly bipartisan" bill aimed at preventing Internet providers from speeding up, slowing down or blocking Web sites. But he'll only cooperate, he said, "provided such action fully protects consumers, does not undercut the FCC's role and leaves the agency with flexible, forward-looking authority to respond to the changes in this dynamic broadband marketplace."

It's a subtle critique of a bill proposed by Sen. John Thune (R-S.D.), the chairman of the Senate Commerce Committee, and Rep. Fred Upton (R-Mich.), the head of the House Energy and Commerce Committee. That legislation would enshrine many of the FCC's regulations into new law. But there's a catch: It would also strip the FCC of some of its powers. And for Democrats like Nelson, that's a non-starter.

"Nelson's comments draw the battle lines," said Paul Gallant, a telecom analyst at Guggenheim Securities. "He's definitely open to moving a bill, but he wants to keep some type of safety-net power for the FCC over ISPs. Republicans don't like open-ended authority for agencies. So, game on."

The Thune-Upton bill would prohibit the FCC from regulating Internet providers using the same tool it uses to police legacy phone service — Title II of the Communications Act. Consumer advocates, Web companies and President Obama all pushed hard for Title II last year, while Internet providers warned it would hurt their ability to offer faster, better service.

The Republican-backed proposal would also limit the FCC's authority to police Internet providers under another part of the law known as Section 706. The FCC has used 706 to knock down state limits on city-run Internet services, and some lobbyists want the agency to flex that muscle even more.

Together, those provisions would make it difficult for the FCC to protect consumers on the Internet, consumer groups say. Republicans argue that their bill would give the FCC ample authority to enforce net neutrality without granting it a blank check.

"I think that you could provide some clear direction for the FCC so that you eliminate some uncertainty and ambiguity, and still retain a lot of authority," Thune told reporters Thursday.

But a spokesman for Nelson warned Wednesday that the FCC needs to be able to respond to future changes in technology. That Nelson is drawing lines in the sand doesn't bode well for the Republican proposal. And Nelson isn't the only Democrat who's balking.

"We don't have a meeting of the minds on the basics of the legislation," Sen. Brian Schatz (D-Hawaii) told National Journal. "I think we ought to be direct with each other about what's realistic."

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