But the two Republican members of the commission said they disagreed with much of the substance of the rules, which would generally prohibit broadband service providers from impeding traffic online.

The proposed regulations, issued Thursday, largely codify the agency's 2005 policy statement providing that Web users are entitled to access all lawful content, applications and services, and that they can attach devices to the network. The FCC also proposed rules codifying two additional principles -- nondiscrimination and transparency.

"Given the potentially huge consequences of having the open Internet diminished through inaction, the time is now to move forward with consideration of fair and reasonable rules of the road -- rules that would be enforceable and implemented on a case-by-case basis," Genachowski said Thursday. "Indeed, it would be a serious failure of responsibility not to consider such rules, for that would be gambling with the most important technological innovation of our time."

Republican Robert McDowell countered that the FCC has no reason to impose new rules now. "I do not share the majority's view that the Internet is showing breaks and cracks, nor do I believe that the government is the best tool to fix it," he said.

While neutrality advocates largely cheered the FCC's decision to open a rulemaking procedure, some expressed wariness about possible loopholes. "It's going to take us a little bit of time to dive deep into the language and make sure there's nothing in here that's of concern," said Derek Turner, research director at Free Press.

The 107-page notice of rulemaking carves out an exception to neutrality rules for reasonable network management practices. That exception could end up being very broad -- encompassing not only spam and malware, but also material like pirated music. "In order for network openness obligations and appropriate enforcement of copyright laws to coexist, it appears reasonable for a broadband Internet access service provider to refuse to transmit copyrighted material if the transfer of that material would violate applicable laws," the notice of proposed rulemaking states.

The potential rules drew intense support and criticism in advance of Thursday's vote. Some opponents argued that the FCC shouldn't even be considering new rules at this time, while others said that neutrality regulations could discourage broadband investment.

Commission member Mignon Clyburn criticized the lobbying strategies in her comments. "Unfortunately, some parties seem to prefer radioactive rhetoric and unseemly and unbecoming tactics," she said. "Such an approach may yield headlines, but will not yield positive results with me."

The FCC will accept initial comments until Jan. 14 and replies until March 5. Even if the agency ultimately adopts the rules, Internet service providers are certain to challenge them in court. Among other arguments, telecoms and cable companies will likely say that the FCC has no authority to regulate the Internet -- an opinion that McDowell endorsed Thursday. "I also disagree with the premise that the Commission has the legal authority to regulate Internet network management as proposed," he said.

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