Thomson Reuters
FILE PHOTO: U.S. Secretary of State Rex Tillerson meets with Russian Foreign Minister Sergey Lavrov in Washington
"The recent reports that foreign nationals with suspected ties to the Russian government sought to influence the 2016 election through social media advertisements are deeply concerning and demand a response," the lawmakers wrote.
"Social media platforms offer the ability to target millions of users based upon a wealth of highly-detailed information," they continued. "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."
The letter was signed by Democratic Sens. Martin Heinrich, Ron Wyden, Tom Udall, Elizabeth Warren, Maggie Hassan, Ben Cardin, Al Franken, Jack Reed, Ed Markey, Chris Van Hollen, Sherrod Brown, Cory Booker, Bob Menendez, Catherine Cortez Masto, Sheldon Whitehouse, and Kamala Harris.
Democratic Reps. John Sarbanes, John Conyers, Elijah Cummings, and Derek Kilmer also signed the letter.
Looking ahead to the 2018 elections, the Democrats called on the FEC to implement improved disclosure standards "to ensure voters have the information they need to evaluate political advertising" - whether it appears on TV, the internet, or elsewhere.
"There is no reason to believe this behavior will stop in future elections," they said.
The FEC last approved regulations governing certain types of internet communications by political committees and campaigns in March 2006, when it amended its rules to include paid online ads as a form of ''public communication.''
FEC spokeswoman Judith Ingram declined to comment on the letter. But she said that the FEC "has put out notices of proposed rulemaking in this area, and last week voted to reopen the comment period in connection with a potential rulemaking."
Facebook acknowledged to CNN last week that it still does not know the extent of Russia's advertisement purchases during the election - or whether these unidentified ad buys are still on the site.
It is unclear whether Facebook could be held liable for political ads purchased by foreign nationals during the election, which is illegal under the Federal Elections Campaign Act.
Robert Mueller, the special counsel leading the FBI's investigation into Russia's election interference, reportedly obtained a search warrant for records of the "inauthentic" accounts Facebook shut down and the targeted ads they purchased. Legal experts said the warrant means Mueller believes that he will be able to obtain enough evidence to charge specific foreign entities with a crime.
The Democrats did not refer to President Donald Trump by name in their letter. But they wrote that the commission's "own plain language guidance on the matter makes clear that it is also illegal 'to help foreign nationals violate'" the ban on contributions from foreigners - including in the form of advertising - to US political campaigns.
"Those found in violation may be subject to 'fines and/or imprisonment,'" the letter says.
The Democrats also asked the FEC to consider "what measures of coordination can be readily utilized" to "monitor illicit coordination between a campaign and a third party political spender, including foreign actors."
Rep. Adam Schiff told MSNBC earlier this month that he wanted to know how sophisticated the Russian-bought ads were - in terms of their content and targets - to determine whether they had any help from the Trump campaign.
Facebook has since confirmed that Russia-linked groups went further than ad buys and memes, and tried to organize anti-immigrant, anti-Hillary Clinton rallies in Texas and Idaho. One Russia-linked group successfully organized pro-Trump "flash mobs" across Florida during the campaign, The Daily Beast reported on Wednesday.
Congressional investigators have reportedly been frustrated by what they see as Facebook's lack of transparency about data that could be helpful in their probe of Russia's election interference. In an attempt to gather more information, the Senate Intelligence Committee plans to hold a public hearing next month focusing on how Russia used social media to try to manipulate voters.