After Facebook disclosed earlier this month that Russia had used its ad platform to promote propaganda, politicians quickly called for closer scrutiny of the company’s ad system. Today, in a letter to the Federal Election Commission, a group of Congressional Democrats called for new rules to keep foreign influence out of American voters’ social media feeds.

“Foreign political activity is a direct assault on federal election law.”

Citing the Facebook controversy, the letter, signed by Democratic senators and representatives, asks the agency to consider guidelines that would help stop similar advertising campaigns in the future. “Foreign political activity is a direct assault on federal election law and the integrity of our elections,” the letter reads.

The letter, signed by Sens. Kamala Harris and Al Franken, among several others, calls for the agency to develop new best practices to prevent foreign influence on American advertising. It also asks the agency to consider what the most vulnerable media and technology companies can do to prevent “hostile actors” from advertising on their platforms.

When it disclosed the Russian ad buys, Facebook said it was experimenting with new ways to detect similar activity in the future. But controversy has continued to follow the company in recent weeks. The company has even reportedly handed over some Russia-linked ads to Robert Mueller, who is investigating possible ties between the Trump campaign and Russia.

“We are fast approaching the 2018 election cycle,” the letter reads. “As such, it is imperative the Federal Election Commission begin this effort in earnest.”