WASHINGTON – As more people use their Internet connections to watch movies and TV shows, fears have grown that broadband providers such as AT&T and Comcast will become online gatekeepers, using their control of the Web to block online video competitors like Netflix and protect their own pay-TV businesses.

That possibility is at the heart of a scheduled vote by the Federal Communications Commission today on new rules to govern how broadband providers manage traffic over their networks. The vote is a pivotal moment in the debate over “network neutrality.” The outcome could affect the ability of Silicon Valley giants such as Netflix and Apple to deliver movies and TV shows online for a fraction of the cost of a cable subscription.

FCC Chairman Julius Genachowski now has the three votes needed for approval, despite firm opposition from the two Republicans on the five-member commission. Genachowski’s two fellow Democrats said Monday they’ll vote for the rules, though they consider them too weak.

At the other end of the spectrum, Republican FCC members and GOP lawmakers say the Web has thrived in the absence of extensive federal regulations and should be left alone.

Genachowski is trying to settle a years-long battle that’s pitted Web companies that want unfettered access to customers online against telecommunications giants that are loath to cede control over networks on which they’ve spent billions of dollars.

Genachowski’s plan would bar broadband operators from blocking access to any legal website or application, or from transmitting traffic over the Web in a discriminatory way. They could, for example, degrade the quality of an online video website that threatens to draw customers away from their own video or cable offerings.

But the plan would give broadband companies a green light to charge heavy Web users more than casual ones. People who stream a lot of Netflix or YouTube video to their laptops or TVs might pay more for their Internet service than those who only send e-mail or read news.

Such “usage-based pricing” plans could give broadband companies a better return on investment and encourage them to beef up their networks, Genachowski said.

But the plan has drawn mixed reaction from Silicon Valley.

Several venture capitalists and entrepreneurs, including Craigslist’s Craig Newmark, said it struck a sensible balance between keeping the Internet open to users and entrepreneurs without saddling broadband operators with burdensome rules that might dissuade them from upgrading networks.

The big broadband providers, meanwhile, have held their fire, relieved that Genachowski backed off his earlier push to regulate them under the much-stricter area of telecommunications law that governs telephone companies.

But the proposal has drawn fire from public interest groups and the Open Internet Coalition, which represents Silicon Valley interests such as Skype and eBay. They say it would give network operators too much room to charge some sites for faster speeds, leaving sites that don’t pay extra in slower lanes.

Such a system, they say, would put online startups that lack deep pockets to pay at a severe disadvantage, hurting innovation on the Web.

Critics also say any rules should apply equally to wired and wireless broadband; Genachowski’s plan would mostly exempt wireless service from the new rules.