WASHINGTON Two Democratic senators began circulating a letter to gain support for legislation mandating disclosure for online political advertising the same day that Facebook announced it would implement its own self-imposed rules for disclosing advertising on the site.

HuffPost obtained a Dear Colleague letter sent by Sens. Mark Warner (D-Va.) and Amy Klobuchar (D-Minn.) on Thursday detailing plans to mandate the disclosure of online political advertising. The effort comes after Facebook revealed that the Russian Internet Research Agency purchased advertisements through its programmatic ad selling platform during the 2016 election to promote President Donald Trumps candidacy and advocate anti-immigrant and anti-Muslim issues that were central to his campaign.

Facebook CEO Mark Zuckerberg announced on Thursday that the massive social media platform would institute its own rules requiring disclosure of advertisements on the site. Those rules would be developed by Facebook and would not be subject to any public oversight. Warner and Klobuchar hope that their legislation would impose actual legal regulation of all online political advertising on large digital platforms.

The particulars of the nascent legislation would require digital platforms with more than one million users to maintain a public file on all electioneering communications purchased by a person or a group that spend more than $10,000 in aggregate for all online advertising. This means that platforms like Facebook , Google , Twitter and Snapchat would be required to host disclosed political advertising on their respective sites. The future legislation is based on the public files that television broadcasters are required to maintain for political advertisements.

A combination of FEC [Federal Election Commission] and FCC [Federal Communications Commission] rules currently create a robust disclaimer and public access regime for political advertisements disseminated by broadcasters, cable, and satellite providers, the letter reads. However, a provider like Facebook, the nations largest digital platform whose 210 million American users are at nearly ten times as large as the subscriber base of the largest cable or satellite provider, and whose daily active user base is nearly as large as the most-watched television broadcast in U.S. history faces significantly fewer obligations.

Each digital advertising file would contain a digital copy of the advertisement, a description of the audience targeted, the number of views generated, the dates and time of publication, the rate charged by the online platform and the purchasers contract. The digital platforms hosting the advertisements would also be required to make reasonable efforts to ensure that the ads purchased were not paid for, directly or indirectly, by foreign nationals or foreign governments.

John Wonderlich, executive director of the pro-transparency Sunlight Foundation, called the legislation, a fairly direct application of analog rules to the digital arena, which is exactly the right starting point, even if it leads us quickly to more complex questions like what counts as an ad, and how will enforcement work.

Paul S. Ryan, a campaign finance lawyer with the grassroots accountability organization Common Cause, said the legislation is an important step toward much-needed transparency for online political advertising. He added that it would enable Common Cause and its membership, to watchdog such advertising for compliance with campaign finance disclosure laws and prohibitions on foreign meddling in U.S. elections and to file complaints on violations with the Department of Justice and FEC.

The FEC stalled on implementing disclosure rules for online political advertising back in 2014 after the three Republican commissioners voted against new rules.

While legislation has not yet been introduced in the House, Rep. John Sarbanes (D-Md.), the head of House Democrats democracy taskforce, told HuffPost in a statement, Congress should also get in the game by enacting appropriate legislative reforms to stop hostile actors from threatening the integrity of our elections.

One key area where the legislation would pull back the curtain is the undisclosed and mysterious political advertising placed on online platforms. For example, in 2016 the Trump campaign carefully targeted specific messages to limited groups on Facebook in an effort to depress turnout among demographic groups that were more likely to turnout and vote for Democratic candidates like Hillary Clinton.

A report by Bloomberg Businessweeks Joshua Green and Sasha Issenberg indicated that the Trump digital operation ran dark post advertisements on Facebook that would only be seen by the intended targets. One of those ads used audio of Clintons comments about super predators from 1996 in an animation that was only targeted to certain African-American voters. [O]nly the people we want to see it, see it, Brad Parscale, the head of Trumps digital operation, told Bloomberg Businessweek.

Keegan Goudiss, a partner at the digital advertising firm Revolution Messaging and director of digital advertising for the presidential campaign of Sen. Bernie Sanders (I-Vt.), said that any digital public filing system, whether self-imposed by Facebook or written into legislation, would need to be better than the slow and complex public files reported by broadcasters.

The biggest thing to me is lets not forget about how hard the FCC file is, Goudiss said. Lets use this as an opportunity to have better disclosure overall.

Goudiss still has big questions about the details of any disclosure regime that would come from Facebook itself or be imposed through legislation passed by elected officials. In particular, he wants to know whether the burden for maintaining and reporting the public files would be placed on the digital platforms or the purchaser of the advertisements. If the burden was placed on the purchasers, that would mean a lot more time on administrative tasks that were going to have to focus on.

Another question is whether the public file would list the original purchaser of the advertisement a campaign or outside group or the agency paid by the purchaser. In their FEC disclosures, campaigns often solely list the agency that they have paid to make digital advertising purchases.

This means that the actual spending on digital platforms, whether the ads were bought on Facebook or Google or Snapchat, is not disclosed. Goudiss thinks that real disclosure would only occur if the campaign or other group were listed as the buyer and not just the agency making the purchases.