The controversial move would give the FCC sweeping new powers over Internet providers.
Several liberal lawmakers want to apply utility-style regulations to Internet service providers.
Sen. Edward Markey, a Massachusetts Democrat, collected signatures for a letter urging the Federal Communications Commission to regulate the Internet like the telephone system.
Democratic Sens. Chuck Schumer of New York and Al Franken of Minnesota, as well as independent Sen. Bernie Sanders of Vermont, have signed on, aides confirmed. The lawmakers have planned a Tuesday morning press conference with Internet advocacy groups.
In the letter, the senators argue that stronger authority is necessary to enact strong net-neutrality rules to prevent broadband providers such as Comcast from manipulating Internet traffic to favor giant corporations.
"Broadband is a more advanced technology than phone service, but in the 21st century, it performs the same essential function," the senators write in a draft of the letter.
"Consumers and businesses cannot live without this vital connection to each other and to the world around them. Accordingly, it would be appropriate for the FCC to reclassify broadband to reflect the vital role the Internet plays in carrying our most important information and our greatest ideas."
Because of a decision during the Bush administration, the FCC currently regulates broadband Internet as an "information service" under Title I of the Communications Act. Reclassifying broadband as a "telecommunications service" under Title II of the law would immediately give the agency sweeping new powers.
The FCC enacted net-neutrality rules in 2010 under the weaker legal authority, but a federal court struck them down earlier this year. The FCC is trying to rework the rules in a way that can survive future court challenges, but the proposal from Chairman Tom Wheeler has prompted a massive public backlash because it would allow Internet providers to charge websites for special "fast lanes" as long as the agreements are "commercially reasonable."
In their letter, the liberal senators warn that Wheeler's proposal "could fundamentally alter the Internet as we know it."
The only way to enact effective net-neutrality rules that bar online discrimination is to use the agency's power under Title II, the senators write.
Although Democrats have long supported net neutrality, few have explicitly called for the FCC to reclassify the Internet under Title II. Thirty-seven House members of the Progressive Caucus signed a letter supporting Title II in May, but 20 Democrats signed a dueling letter opposing the option.
Republicans and major broadband providers have threatened a legal and political war with the FCC if it tries to reclassify the Internet.
Top Senate Republicans sent a letter to Wheeler in May, warning that "monopoly-era Title II regulations" would stifle investment and undermine his "ability to effectively lead the FCC." House Republican leaders fired off a similar letter the following day.
In a blog post Monday, the National Cable and Telecommunications Association, the lobbying group that represents Comcast and other broadband providers, urged the FCC to reject the "extreme voices" calling for reclassification.
Utility-style "common carrier" regulations would "deter ongoing investments and innovation" and would not even survive in court, the cable group warned.
Under Title II, the FCC has broad regulatory powers, including the ability to control prices and determine which customers a company has to serve. But the commission can also decide to waive any requirements under the provision.