A coalition of Senate Democrats asked the Federal Election Commission on Monday to rewrite its rules so that foreign interests are unable to influence federal elections.

Sen. Al Franken and 14 of his colleagues argued in a letter that FEC guidelines on the political operations of foreign companies “have grown out of date.”

“We write with a simple request: that the commission use this process to repair and strengthen protections against foreign influence of American elections,” the Minnesota Democrat wrote in the letter, which was also signed by Majority Whip Dick Durbin (Ill.), Judiciary Chairman Patrick Leahy (Vt.) and Democratic Senatorial Campaign Committee Chairman Bob Menendez (N.J.).

Franken’s letter comes less than two weeks after he sent a similar letter to FEC Chairman Matthew Petersen, a Republican. Franken wrote then that he was “profoundly concerned by recent reports that foreign corporations are indirectly spending significant sums to influence American elections through third-party trade groups.”

Democrats have accused the U.S. Chamber of Commerce of illegally directing dues from foreign entities to a $75 million advertising campaign that bashes the majority party’s candidates. The charge, originally levied Oct. 5 by the left-leaning blog ThinkProgress, accused the chamber of “likely skirting long-standing campaign finance law that bans the involvement of foreign corporations in American elections.”

But Democrats have been unable to offer evidence to back up the accusation, a point that Allison Hayward, who works at the libertarian Center for Competitive Politics, emphasized in a statement Tuesday.

“Federal law and FEC regulations already mandate ample restrictions, including a prohibition on foreign nationals playing any role in political spending decisions,” she said. “This is a manufactured conspiracy theory unjustified by any showing of a real problem.”