Fifteen Democratic senators signed comments to the Federal Election Commission in support of new rules for online political advertisements after Russian efforts to interfere in the 2016 presidential election.

Russia-linked groups are believed to have purchased political ads on platforms including Facebook and Google as part of a broader influence campaign. The ads appeared to come from domestic advocacy groups and played up controversial topics to sow conflict.

"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," the Democratic senators wrote.

"The efficacy of these efforts will depend, in no small part, on whether our election laws remain riddled with loopholes that enable online political advertising to escape the same transparency and accountability requirements of other forms of large-scale political advertising.

The proposed FEC rules would expand disclosure requirements, requiring companies to share more information about who purchased the online ads. Online ads would need to have the same kind of notification messages as political ads for television.

Monday is the last day of the public comment period for the FEC rule.

The comments were spearheaded by Democratic Sens. Amy Klobuchar (Minn.), Mark Warner (Va.), and Claire McCaskill (Mo.) and cosigned by Democratic Sens. Michael Bennet (Colo.), Richard Blumenthal (Conn.), Sherrod Brown (Ohio), Bob Casey (Pa.), Dianne Feinstein (Calif.), Al Franken (Minn.), Tim Kaine (Va.), Joe Manchin (D-W.Va.), Ed Markey (Mass.), Bill Nelson (Fla.), Jack Reed (D-R.I.) and Chris Van Hollen (Md.).

Klobuchar and Warner have also introduced legislation that would expand disclosure requirements for online political ads.