US lawmakers Thursday unveiled legislation to require disclosure of the source of many online political ads, a move aimed at preventing a recurrence of Russian social media manipulation in the 2016 election.
"First and foremost this is an issue of national security -- Russia attacked us and will continue to use different tactics to undermine our democracy and divide our country, including by purchasing disruptive online political ads," said Senator Amy Klobuchar, who introduced the bill with fellow Democrat Mark Warner and Republican John McCain.
The legislation follows news that Russian-backed entities used online platforms to spread disinformation during the 2016 campaign, aiming to help Republican Donald Trump defeat Democratic challenger Hillary Clinton.
"In the wake of Russia's attack on the 2016 election, it is more important than ever to strengthen our defenses against foreign interference in our elections," McCain said in a statement.
"Unfortunately, US laws requiring transparency in political campaigns have not kept pace with rapid advances in technology, allowing our adversaries to take advantage of these loopholes to influence millions of American voters with impunity."
The Honest Ads Act would require online platforms with at least 50 million users to maintain and disclose information on spending of at least $500 for ads for candidates or legislative issues, applying rules that are similar to those for television and radio.
A similar measure is being introduced in the House of Representatives with bipartisan backing.
The lawmakers acknowledged some resistance from major tech firms, but said talks were ongoing.
"It's our hope the social media companies, the platform companies will work with us," Warner said.
"The companies that we're talking about are really iconic American companies... but these companies in many ways rely upon the trust of all of us who use these platforms."
The New York Times reported that some major tech companies opposed the bill because it would add a new level of regulation and may not apply to divisive messages that do not mention specific candidates or issues.
Facebook vice president and chief privacy officer 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."
Internet Association president Michael Beckerman said the group which represents major online firms was "reviewing the legislation and look forward to further engagement with the sponsors."
Berin Szoka of the lobby group TechFreedom called the measure well-intentioned but said it would be complicated to implement.
Szoka said US election law already bans foreign nationals from spending on campaigns.
"It's fair to expect online ad platforms to do more to identify ads funded by foreign sources, but imposing vague standards for intermediary liability is always a bad idea," he said
"We also don't want ad platforms to be so unsure of their potential liability that they make it harder for American citizens, especially those of foreign origin or living overseas, to exercise their free speech rights online."
Lawmakers are investigating how foreign entities used Facebook, Google and other online platforms to sway sentiment in 2016.
The social network handed to Congress about 3,000 Russia-linked ads that appeared to use hot-button issues to turn people against one another ahead of last year's US election.
Many of these were so-called "dark ads" targeted at specific groups and which could not be viewed by the public.