Since the Supreme Court struck down campaign finance legislation in the Citizens United decision, groups have not been required to disclose the sources of ads advocating issues. Independent committees may spend an unlimited amount of money on campaigns as long as they do not coordinate or donate directly or in-kind to candidates or parties.

The radio ads will run May 24 through June 1 in Chattanooga and Nashville, Tenn., and Bangor and Portland, Maine, and cost the group just under $100,000.

While the League calls on senators to support full disclosure of ads, the group declined to disclose its own donors for an anti-Sen. Scott Brown (D-Mass.) advertisement reported on last year by the Boston Globe. Instead, the group indicated all donors were listed in its annual report.

League President Elisabeth MacNamara told The Hill the group has had “a running discussion with The Boston Globe,” disagreeing on whether the ad was “against Sen. Scott Brown.” She instead indicated the ads were issue ads discussing the Environmental Protection Agency and emphasized the organization does not take part in “election advertising.”

Similar ads on clean air also ran in Missouri since Sen. Claire McCaskillClaire Conner McCaskillSenate Dems lock in million in TV airtime Why does Congress keep playing political games on FBI oversight? Red-state Dem tells Schumer to 'kiss my you know what' on Supreme Court vote MORE (D-Miss.) "cast the same vote" as Brown, she said.

“Our concern with electioneering ads — which our ads were not — is that folks can assess these ads on credibility and that you know you who's speaking to you. With these groups, there is no history, there is no track record, and there is no way to know who actually is speaking. With the League, you know who's speaking,” she said, adding the LWV is a well-known organization and has built credibility.

There are currently versions of a “Disclose Act” in both the House and Senate, which would require additional transparency in campaign activities. For instance, both bills contain a “stand by your ad” provision, which would require the heads of the groups to include an “I approve this message” disclaimer in campaign ads.