Sens. Amy Klobuchar (D-Minn.) and Mark R. Warner (D-Va.) introduce the Honest Ads Act  in Washington on Oct. 19. (Photo by Michael Reynolds/EPA-EFE/REX/Shutterstock)

Facebook, Google and Twitter are one step closer to facing new regulations that would place transparency requirements on a core aspect of their businesses: advertising.

With support from Sen. John McCain (R-Ariz.), Democratic Sens. Amy Klobuchar (Minn.) and Mark R. Warner (Va.) unveiled a bill Thursday that would mandate greater disclosure of political ads that run on large Internet platforms. The proposed legislation was crafted as a response to a Russian-financed propaganda campaign that ran on an array of Web platforms to spread disinformation and discord during the 2016 election.

While the proposed legislation was expected for weeks, it remained unclear whether the bill would attract Republican support other than McCain's and whether tech companies would object to rules that would entangle their ad operations.

“Who wouldn't want to know if the ad that's appearing next to your story was actually paid for by a foreign power?” Warner said. “I don't know what opposition there would be to that kind of disclosure.”

During the briefing, Klobuchar and Warner said that the content of online ads, and the people who fund them, remain unknown to the public and because of that the democratic process is vulnerable to meddling. And beyond foreign powers attempting to interfere in U.S. elections, they emphasized that outdated laws have failed to grapple with the evolution of massive online platforms, and the way political speech travels on the Web. The proposed legislation, they said, would be a much needed corrective.

McCain said in an interview on Thursday that his bill was simply the next frontier in a decades-long fight to force more transparency about money in politics.

“For 25 years, I’ve been fighting for full disclosure of what’s going on. This is a logical step in that direction,” he said. “There’s no departure from my past.”

McCain would not say whether his support might pave the way for more Republicans to come on board. But at least one of McCain’s allies in the Senate said he is “very interested” in the proposal. “Social media advertising had to be regulated, it’s the wild wild west,” said Sen. Lindsey O. Graham (R-S.C.). When it comes to disclosing who pays for online advertisements, he continued, “you’ve got to try to apply the same rules you would to radio and TV.”

The bill would require digital platforms with more than 50 million monthly viewers to create a public database of political ads purchased by a person or group who spends more than $500. The public file would include the ad, a description of the targeted audience, the number of views it generated, the date and time it ran, its price, and contract information for the purchaser

The proposal would create a reporting system akin to the one required of television stations by the Federal Communications Commission – although it would go further by requiring the disclosure of the ads themselves, not just the amount spent to run them.

Warner described the legislation as “common sense light touch regulation.” And he said he hopes that tech companies will work with him and Klobuchar to ensure its passage.

Riva Sciuto, a spokeswoman for Google, said in a statement, “We support efforts to improve transparency, enhance disclosures, and reduce foreign abuse. We're evaluating steps we can take on our own platforms and will work closely with lawmakers, the FEC, and the industry to explore the best solutions.”

Facebook's vice president of U.S. public policy, Erin Egan, said in a statement: “We stand with lawmakers in their effort to achieve transparency in political advertising. We have already announced the steps Facebook will take on our own and we look forward to continuing the conversation with lawmakers as we work toward a legislative solution.”

The co-sponsors of the bill touted the endorsements of several civil society groups, including the Campaign Legal Center, the Sunlight Foundation, Issue One, the Brennan Center of Justice, Common Cause and Public Citizen. But some are skeptical that the Honest Ads Act will gain enough political support, given the Republican-controlled Congress and potential attacks from free speech advocates and those who oppose the creation of new campaign finance regulations.

A companion bill in the House was also introduced Thursday by Reps. Derek Kilmer, a Democrat from Washington, and Mike Coffman, a Republican from Colorado.