A group of 11 senators — including Ron Wyden (D-Ore.), Chuck Schumer (D-N.Y.) and Elizabeth Warren (D-Mass.) — urged Wheeler in a letter to drop a controversial approach allowing Internet-service providers like AT&T and Verizon to charge companies for faster delivery of their content. The Democrats’ complaints echoed those of Google, Facebook and other tech giants, which labeled Wheeler’s proposal a “grave threat” to the Internet on Wednesday.

“Sanctioning paid prioritization would allow discrimination and irrevocably change the Internet as we know it,” the senators wrote. “Small businesses, content creators and Internet users must not be held hostage by an increasingly consolidated broadband industry.”

( Also on POLITICO: Net neutrality pressure mounts)

The statements add to the pressure on Wheeler as the FCC hurtles toward a May 15 vote on his proposal. As opposition mounts, commissioners Jessica Rosenworcel, a Democrat, and Ajit Pai, a Republican, have called for the chairman to postpone consideration of the plan.

Wheeler on Friday sought to tamp down criticism, calling the vote a necessary first step to begin the process of crafting net neutrality rules that would ensure all Web traffic is treated equally.

“The action that I propose to take May 15 is to make that proposal public for the first time and invite comment by the American people,” he wrote in a letter to consumer and advocacy groups. “We will use the notice and comment procedure mandated by law to make the proposal available to all and to actively solicit comments, suggestions and other input.”

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Wheeler said he would “not allow some companies to force Internet users into a slow lane so that others with special privileges can have superior service” and added that all options “remain on the table,” including reclassifying Internet service as a communications utility — an approach favored by net neutrality advocates as more robust but opposed by telecom companies.

Behind the scenes, Wheeler has practically worn out the carpet on the top floor of FCC’s headquarters, as he attempts to forge a compromise that might win over his fellow Democratic commissioners, Rosenworcel and Mignon Clyburn, who both favor a stronger net neutrality proposal, according to agency sources.

Wheeler is expected to produce a new draft plan on Monday, though the details aren’t yet clear, the sources said. Aides to Rosenworcel and Clyburn said their offices are waiting to see what changes Wheeler incorporates. Without the support of the two Democrats, the chairman faces the possibility of a stunning political defeat.

A collection of trade and advocacy groups — including the Internet Association, Engine Advocacy, the National Association of Realtors and COMPTEL — sent their own letter to Wheeler Friday calling on him to rethink his proposal.

AT&T, meanwhile, launched a counteroffensive. Executives from the company warned the FCC in a Thursday meeting not to reclassify broadband as a telecommunications service, saying such a step “would ignite multiyear regulatory controversies on a variety of issues,” according to a filing with the commission. Telecoms dislike that approach because they fear new regulations would unfairly restrict their business.

For now, the FCC has extended its comment period on the net neutrality proposal to allow people more time to offer their views. It appears the agency has gotten a large volume of calls. The FCC’s main switchboard on Friday played a message urging those calling about the “open Internet” to email the agency instead.

Alex Byers contributed to this report.