End Citizens United, a liberal political action committee focused on driving big money out of politics, will be endorsing Sen. Heidi Heitkamp, D-N.D., for the upcoming U.S. Senate race.

Tiffany Muller, executive director of End Citizens United, said Heitkamp has been a leader in the fight to reform the campaign finance system, even during her time as attorney general of North Dakota.

"Senator Heitkamp is a champion on the issue and always has been," Muller said. "Just this last spring she helped introduce a bill that would increase transparency and pushback against the dark money in our elections."

The SUN (Sunlight for Unaccountable Nonprofits) Act, which was introduced by Heitkamp and Sen. Jon Tester, D-Mont., would allow the public to know when donors spend more than $5,000 on tax-exempt groups that engage in electioneering and would have no impact on nonprofits that don't engage in election activities.

"Billionaires outside North Dakota who don't know anything about our state shouldn't be able to impact North Dakotans' opinions or how they vote through secret organizations that don't have to disclose their donors. It's one example of the many problems with our campaign finance system, and it needs to change," Heitkamp said in a statement. "Since serving as North Dakota's Attorney General, I've been working to reform campaign finance laws, make them more transparent and give North Dakotans the voice they deserve. I'm proud to serve North Dakota and am honored to have the endorsement of End Citizens United."

Established in 2015, the group's goal is to overturn the 2010 Supreme Court decision that deregulated corporate and union spending for or against specific candidates. According to their website, the group endorses Democratic candidates committed to a campaign finance overhaul. Last month, the group announced that it raised $4 million in the first quarter of 2017 with 100,000 donors who gave an average contribution of $14. It projects it will raise $35 million for the 2018 cycle, surpassing the $25 million it raised for the 2016 elections.

According to a press release, the group has over 3 million members and was the third largest federal Political Action Committee of the 2016 election cycle. There are nearly 4,000 members across North Dakota.