The letter could give the FCC political cover to enact tougher rules.
Harry Reid wants the Federal Communications Commission to know he'll be on the agency's side in a political battle with Republicans.
In a letter to liberal advocacy groups, the Senate majority leader vowed to "lead the fight" to defend net-neutrality regulations "against the inevitable Republican attack."
The Senate's top Democrat said the regulations should give consumers "access to the lawful content they want when they want it" and should "ensure that priority arrangements that harm consumers are prohibited."
Reid's letter could reassure FCC Chairman Tom Wheeler that he has the political support he needs to ignore the Republican outcry and enact strong net-neutrality regulations.
The FCC first enacted net-neutrality rules in 2010 that barred Internet service providers from blocking websites or "unreasonably" discriminating against any traffic. A federal court struck those rules down earlier this year, and Wheeler is now trying to rework the regulations in a way that can survive court challenges.
His proposal has sparked a massive backlash because it could allow Internet providers to charge websites for faster service as long as the agreements are "commercially reasonable."
Liberal groups and some Democratic lawmakers have urged Wheeler to regulate Internet service providers under the same authority that the FCC uses for telephone companies. Reclassifying Internet providers as utilities under Title II of the Communications Act would give the FCC sweeping power to enact tough net-neutrality rules, but it would prompt a major battle with industry groups and congressional Republicans.
Most Republicans believe net-neutrality rules are unnecessary and burdensome, but they would be apoplectic if the FCC treated the Internet like a utility. House and Senate GOP leaders have sent letters to Wheeler warning him not to apply "monopoly-era" rules to the Internet, maintaining it would kill investment in broadband networks.
Under regulatory procedure, Congress will have an opportunity to vote to overturn any rules the FCC enacts.
Reid's letter did not urge the FCC to use its authority under Title II. But he acknowledged that liberal groups are pressing the FCC on the issue and said he would support "any Open Internet rules" the FCC enacts.
David Segal, the executive director of the advocacy group Demand Progress, said the letter shows Senate Democrats will defend the FCC if it uses the Title II option and that Republicans would likely fight the rules no matter what authority the FCC uses.
"So it leaves them to decide this important issue on the merits," Segal said, noting that he and other advocates have been clear that they believe Title II is the right path.
Reid sent the letter, which is dated Monday, in response to a letter from Demand Progress, Daily Kos, MoveOn, CREDO, and other liberal advocacy groups.
Bloomberg first reported on the letter.