If political advocacy groups want to pour thousands of dollars into ballot measure campaigns they should be required to publicly reveal a list of their top donors, Rep. Mark Mickelson said Wednesday.

The Sioux Falls Republican said that the 2016 election illustrated why the state should ask for more information about the top 50 donors of groups that contribute $25,000 or more a year to ballot measure campaigns.

Out-of-state labor unions, open primary advocacy groups, national payday lenders and Americans for Prosperity, the primary political arm of the Koch brothers, each spent hundreds of thousands of dollars supporting or opposing issues on last year's ballot.

If they want to continue doing so, South Dakota residents have a right to know who's footing the bill for all the television ads and campaign mailers, Mickelson said.

"The big money has started to come in and (influence) the initiated measure process," Mickelson said. "Some of them have chosen to make South Dakota their playground and that has negative consequences for those who call this state their home."

The proposal mirrors a bill Mickelson brought during the 2017 legislative session. That proposal passed the House but failed in the Senate.

He is also bringing a separate proposal to ban out-of-state funds to ballot measure campaigns entirely.

Advocacy groups were quick to oppose the measure, saying it would prevent free political speech and could land the state in court. In its comments about the draft proposal, the Legislative Research Council indicated that it could pose constitutional problems for the state if approved.

The U.S. Supreme Court has previously ruled against limits on donations to ballot measure campaigns, saying the gifts constitute political speech.

Ben Lee, state director of Americans for Prosperity, said the measure would have a "chilling" impact on private donations to the group or to political speech.

"This was or would be a restriction on South Dakotans' rights to free speech and association," Lee said.

And where individuals feel comfortable donating to political advocacy issues now, they might choose not to do so in the future if they felt their boss or family member of a different political view might find out.

"All of a sudden they're going to worry about other people will think," Lee said.

Under the proposal, those who are late to disclose top donors would be subject to a $5,000 fine that would benefit the state's general fund. Groups that choose not to disclose would have to pay 25 percent of the total amount they gave in the last calendar year, while leaders could face misdemeanor charges.

Mickelson must collect 13,871 valid petition signatures to place the proposal on the 2018 ballot.

Follow Dana Ferguson on Twitter @bydanaferguson, call 605-370-2493 or email dferguson@argusleader.com