As congressional lawmakers investigate Russia's role in the 2016 presidential election, they're also evaluating how tech giants may have played a part in spreading propaganda, fake news and other misinformation. And that scrutiny could soon lead to changes in what political ads on their platforms are required to disclose.

In the last several months, Facebook Inc. (FB) - Get Report , Alphabet Inc. (GOOGL) - Get Report and Twitter Inc. (TWTR) - Get Report have turned over evidence that Russian operatives purchased ads and set up accounts on their sites during the election. Facebook said in September that it had discovered a Russian "troll farm" had purchased $100,000 worth of advertising, while Google said on Monday it had discovered that tens of thousands of dollars worth of ads had been bought by Russian agents on YouTube and other sites. Twitter also recently disclosed that it found more than 200 Russia-linked Twitter bots.

The companies have all committed to look into the issues further, but Facebook (which initially struck down the idea that it could have played a role in swaying the election) has unloaded a slew of new initiatives and requirements in response to the evidence of Russian involvement, including adding workers and policies to improve political ad transparency. Some are interpreting this as a preemptive move, since legislators have indicated that they may soon adopt new disclosure rules for online political ads. 

Calls have intensified for the Federal Election Commission (FEC) to adopt new rules that would prohibit foreign nationals from buying online political ads to influence voters on platforms such as Facebook and Google. On Thursday, Sen. John McCain, of Arizona, signed on to a bill being drafted by Democratic Senators Amy Klobuchar and Mark Warner that would require online political ads to disclose their funding sources. The bill, The Honest Ads Act, would also require digital platforms to archive all political ads sold above certain thresholds, according to Axios. 

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Last month, Democrats in the Senate sent a letter to the FEC, urging them to develop new guidelines for online political ads, including improved disclosure guidelines, similar to how TV ads are required to mention any political sponsors. The Senators signing the letter included Al Franken, Cory Booker, Sherrod Brown and Kamala Harris. 

"Social media platforms offer the ability to target millions of users based upon a wealth of highly-detailed information," the lawmakers wrote. "As we have seen, the low cost of reaching these users equips hostile foreign actors with a powerful new tool for disruption of our democratic process. Therefore, it is incumbent that the Commission take immediate action to preserve the integrity of our election law and our elections."

"There is no reason to believe this behavior will stop in future elections," the lawmakers continued in the letter.

It's likely that this issue will only intensify in the coming weeks, as Facebook, Google and Twitter are scheduled to testify in front of the Senate Intelligence Committee on Nov. 1 as part of a probe into Russian interference during the 2016 election. 

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Editors' pick: Originally published Oct. 11.

The FEC already has some regulations in place for online advertisements, with the FEC updating its guidelines in 2006 to include paid online ads as a form of public communication. But Facebook and Google have been able to avoid some of these regulations for years. In 2011, Google successfully lobbied to be exempt from FEC rules, saying character limits in advertisements would make it difficult for advertisers to adhere by the disclosure guidelines. One year later, Facebook used a similar argument to gain an exemption from FEC rules, saying that such disclosure requirements would be "inconvenient and impractical." 

As a result, the FEC currently views Facebook and Google ads as "small campaign items," putting them in the same category as items such as bumper stickers, pens, t-shirts and other campaign investments for messaging purposes, said Ryan Hagemann, director of technology policy at the libertarian think tank the Niskanen Center.

If the FEC does decide to institute new disclosure regulations, political ads could require a lot more real estate on users' Facebook timelines and Google Search results. 

"I'm not sure how this would actually work in practice, given the limited amount of characters permitted for Facebook ads," Hagemann said. "I would venture to guess that any law mandating disclosures would mean political ads end up being bigger than other ads in an individual's feed in order to accommodate the extra space required for those notifications."

What isn't as clear is how stiffer disclosure rules would impact Facebook, Google and Twitter's relationships with advertisers. Digital political advertising, which includes mobile and social media, search, video and email, totaled roughly $1.4 billion during the 2016 election, according to media tracker Borrell Associates. Of that, roughly 41% of ad dollars went to social media sites, with Facebook earning $390 million and the rest going to Google, Instagram and Twitter, according to Borrell.

Ultimately, any impact would probably be "infinitesimally small," Hagemann said. Just as disclosure requirements haven't prevented lobbying groups or interest groups from advertising on TV, it's unlikely such rules would have a significant effect on social media advertising. 

"I can't imagine the ad purchasers would just decide not to engage with the audience available through platforms like Facebook, Google and Twitter," Hagemann explained. "Any future law or regulation requiring disclosure probably won't deter political agents, whatever their motives, from using the site for outreach and messaging; it may, however, end up having any number of unintended negative consequences we can't yet foresee."

Updated from Oct. 11