A group of 15 Democratic senators on Monday urged the Federal Election Commission to clarify its disclaimer requirements for online ads, a change the agency is discussing after it was revealed Russian entities purchased ads on social media platforms during the 2016 election.

“Over the past year, our country has come to realize the ease with which foreign actors can interfere in our elections, undermining the integrity of — and reducing public confidence in —the electoral process,” the Democrats, led by Sens. Mark Warner of Virginia, Amy Klobuchar of Minnesota and Claire McCaskill of Missouri, told the FEC. “As part of a wide-ranging interference campaign during the 2016 election, Russian operatives used advertisements on social media platforms to sow division and discord, distorting public disclosure and coarsening our political debate."

“The actions undertaken by Russia should not be considered an anomaly; they will be the norm in future elections if we do not take immediate action to improve the transparency and security of our election process," they warned.

Paid political ads that run on TV and radio are required to include disclaimers stating who paid for them, but many of those requirements do not apply to some online paid political ads.

Google and Facebook asked the FEC to exempt them from disclaimer requirements in 2010 and 2011, respectively.

Lawmakers began to highlight the disparity between disclaimer requirements after Facebook revealed a Russian troll farm purchased ads on the platform during the 2016 election. It was later revealed Russia also purchased ads through Twitter and Google.

Facebook, Twitter and Google have since taken steps to improve transparency of online paid political ads. But some members of Congress believe the federal government should take steps to boost transparency.

The FEC first considered modifying disclaimer requirements for online paid political ads in 2011, and re-opened the comment period again last month in the wake of the revelations about Russian meddling in the 2016 election.

The agency is accepting comments through Monday.

“The FEC must close loopholes that have allowed foreign adversaries to sow discord and misinform the American electorate,” the Democrats wrote. “The lack of transparency for digital ads is a threat to our national security. Without change, the misuse of online advertisements during the 2016 election will serve as a template for other foreign powers who with to influence our elections. Failure to act threatens the very foundation of our democracy.”