WASHINGTON — Twitter, Reddit, Tumblr, and hundreds of other businesses and organizations are calling on FCC Chairman Ajit Pai to reverse course and scrap his plans to repeal most of the net neutrality rules.

Their letter, made public on Monday, is the latest effort among advocates to sound the alarm about the pending FCC vote on Dec. 12. The commission’s Republican majority is expected to repeal Pai’s proposal that would cancel rules that ban internet providers from blocking or throttling traffic, or from selling “fast lanes” so content companies can obtain speedier access to the consumer.

“Without these rules, internet service providers will be able to favor certain websites and e-businesses, or the platforms they use to garner new customers, over others by putting the ones that can pay in fast lanes and slowing down or even blocking others,” the letter stated. “Businesses may have to pay a toll just to reach customers. This would put small and medium-sized businesses at a disadvantage and prevent innovative new ones from even getting off the ground. An internet without net neutrality protections would be the opposite of the open market, with a few powerful cable and phone companies picking winners and losers instead of consumers.”

They added, “The current rules provide the protections necessary to protect net neutrality and ensure the internet remains a free and open marketplace that encourages innovation and supports robust competition.”

They also are calling for the FCC to retain its designation of internet service as a common carrier. That regulatory designation is what allowed the FCC to establish a robust set of rules for broadband in 2015, when Democrats were a majority on the commission.

Pai and other supporters on the FCC say that the current rules are a burden to business, and have stifled investment and innovation. He is proposing retaining a rule that requires ISPs to disclose their traffic management practices, including when they block or slow a site, and the type of content that is being targeted.