Despite some strong support, the bill faces opposition not only from the tech industry, which considers the reforms too broad, and a threat to its offering services like encryption, but also from some victim advocates who view it as too narrow an approach to combating harms online.

Last year, tech companies reported nearly 70 million images and videos related to online child exploitation. They are obligated to report the material when they become aware of it on their platforms, but they are not required to go looking for it. Companies are generally not responsible for content uploaded by their users, because of a 1990s-era provision in the law known as Section 230.

The new bill, called the EARN IT Act, would carve out an exception to that rule. Companies that don’t follow the recommended standards would lose civil liability protections for that type of content. The legislation would also lower the bar for suing those tech firms.

The bill would create a 19-member commission — including members representing law enforcement, the tech industry and child advocates — to recommend a set of strategies: for instance, how to spot illegal material, categorize it in standardized ways and verify users’ ages. These practices would be subject to approval by the Justice Department and other agencies, as well as Congress and the president.

In an interview, Mr. Blumenthal said the bill reflected the ideas that tech was no longer the fragile sector it was in the 1990s, and that the public would benefit from reasonable rules of the road.