Elections come and go, but the season for political advertising sometimes seems to never end.

Facebook users in California, for example, may have noticed ads in recent months showing a woman with duct tape over her mouth and text that warns: "Legislators are leaving sexual assault survivors from public universities out to dry."

The ads urge readers to sign a petition seeking to add public universities to legislation that would give students at private universities more time to sue their schools over sexual assaults.

Neither the ads nor the website where they lead offer any clue about who is paying for them. Unlike most election season ads about candidates or ballot measures, they don't have to.

Now, California legislators are considering a proposal by Assemblyman Kevin Mullin that would require groups buying such "issue advocacy ads" about legislation to identify themselves and major funders in the same sort of disclaimers required in election campaign commercials.

"This is about well-funded, sophisticated special interests," said Mullin, a San Mateo Democrat.

Proponents say the measure would be the first in the U.S. to address what they see as a burgeoning issue in the world of influence peddling as interest groups seize on the anonymity afforded by the internet and social media advertising.

"I've never seen so many issue ads," said Trent Lange, president and executive director of California Clean Money Campaign, which is sponsoring Mullin's bill.

Critics argue the proposed law would discourage grassroots activism, particularly around hot-button issues.

Current law requires groups lobbying the California Legislature to disclose expenses such as advertising in routine quarterly filings with the secretary of state.

But an ad campaign may be over at that point, and groups do not necessarily have to specify which ads they funded, noted Kati Phillips, spokeswoman for the campaign finance reform group Common Cause California, which supports Mullin's bill.

Chasing down who exactly is behind an advertising campaign to influence legislation at California's Capitol can be a journey through corporation filings, political spending disclosures and anonymous social media pages — all sometimes leading to a dead end.

Mullin's legislation would apply to groups spending more than $10,000 a year on advertisements about pending legislation or regulation. A disclaimer attached to the bottom of such ads would have to include not just the group behind the ad but its top three donors, if any have given more than $10,000. Some political messages — such as emails to a group's members, small buttons or skywriting — would be exempt.

The California Teachers Association and some other labor unions — big spenders in California politics — have opposed the bill, saying it would stifle advocacy.

David Keating, president of the Institute for Free Speech in Washington, D.C., argued that requiring a group to put the names of its top three funders on advertisements could discourage people from supporting campaigns involved in divisive issues.

That would mean fewer ads and awareness campaigns about legislation, he said.

"You're going to get less speech, less information about what's going on in Sacramento," Keating argued.

There are other examples of anonymous ad campaigns this session.

Turn on talk radio or check Facebook in many parts of California, and you may see an ad about pending legislation that would cap the interest lending companies charge on installment loans with rates spiraling into the triple digits.

The commercials are part of a campaign called Don't Lock Me Out California, which argues the proposed law would leave customers with fewer options when they need quick cash in an emergency.

Click on the group's ads on Facebook, where it has spent more than $26,000 to reach Californians, and there is no contact information. The ads do not indicate who is paying for the messages urging Californians to tell their lawmakers to vote against the bill.

However, Federal Communications Commission records show the Online Lenders Alliance, a national trade group that lobbies for the industry, bought time on Sacramento-area radio stations to air the Don't Lock Me Out California ads. The group is not necessarily required to register with the Secretary of State's Office.

Spokesman Andrew Ricci said the alliance has not made any financial contribution to the campaign, which he described as the work of a coalition that includes Online Lenders Alliance members as well as organizations that are not members of the group. He declined to identify other members of the coalition.

Fort Worth, Texas-based Elevate Credit Services reported spending more than $100,000 in the first quarter of the year to influence the state government on top of its fees to lobbyists.

It also reported lobbying on the lending bill. Asked if it was funding the Don't Lock Me Out California campaign, spokeswoman Marian Daniells said only that Elevate Credit Services does not comment on "government relations-related expenses."

Mullin's political advertising bill is AB1217.