Senator Jon Tester is far more likely to appear in the halls of Congress than the corridors of CAA. But the Montana Democrat came to the venerable talent agency earlier this week—not seeking representation, but to represent.

Tester turned out in support of the Oscar-shortlisted documentary Dark Money at a CAA screening that drew a standing room-only crowd. The film directed by Kimberly Reed, a fellow Montanan, investigates the pernicious role of untraceable cash flooding U.S. elections. The subject is an important one to Tester, who just won re-election to a third term in a very tight race that attracted loads of anonymous spending.

“I can’t tell you how many dollars of dark money came into my state yet,” he told the audience during a Q&A. “And we may never know how many dollars came into the state. If it was for me or against me, it’s the same.”

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In an exclusive interview with Deadline, Tester added, “The truth is there were flyers, there were telephone calls and there was a lot of money spent. What the advocates of Citizens United will say is [dollars] were spent to try to inform the voter. That couldn’t be further from the truth. It was spent to try to confuse the voter so when they went to the polls they wouldn’t know who to vote for.”

In the 2010 Citizens United decision he refers to, the U.S. Supreme Court found corporations enjoy free speech rights and can’t be restricted in their independent political spending. But they’re still prohibited from coordinating directly with candidates and campaigns—in theory.

In Dark Money, Reed follows the case of a powerful Republican state legislator in Montana who was accused of letting a nonprofit group—bankrolled by secret corporate cash—run his election campaign.

“The film actually ends up connecting a lot of those dots…in a way that everything comes together in the trial of one corrupt legislator who’s being held accountable for taking a bunch of this dark money,” Reed notes. “Having the bad guy get held accountable for something they did wrong and taking him to court is pretty much the formula for every episode of Law & Order you ever saw.”

Montana has a long bipartisan tradition of trying to keep undue corporate influence out of elections, borne of painful experience when the state’s copper barons used to rule government.

“Back in 1912 [buying legislators] happened literally with bags of money. And Montanans stood up and said, ‘No more of this. We’re not going to let corporations buy our elections,’ and they passed by voter initiative a ban on corporate dollars going into campaigns,” Tester explains. “So now you fast-forward to where we are now, a hundred years later. And I think Montanans still understand the impacts of corporate money in our campaigns and they don’t like it.”

Citizens United has complicated Montana’s efforts to clear dark money out of elections, but the state remains a leader in mandating disclosure of corporate donations in politics. Nothing in the Citizens United decision bars that.

“One of the things we chronicle in the film is this group of Republicans working with a Democratic governor in a state that Trump took by 20 points to create some of the strongest campaign finance laws in the country,” Reed observes. “And that’s a really hopeful story.”

Among the characters in Reed’s film, along with Sen. Tester, is Ann Ravel, a former Democratic commissioner of the Federal Election Commission. She also participated in Monday night’s Q&A, noting that polls suggest Americans overwhelmingly support curbs on corporate spending in elections. Pressure for reform could eventually sweep from the grass roots to Washington, she suggested.

“Once there’s either more of the constitutional amendments or statutory changes in states and in local governments,” she commented, “it’s going to be a way to convince congress—hopefully, to be optimistic—that they need to pass these kinds of rules too.”

Reed noted that’s on the agenda of the newly-seated House of Representatives, which is now under Democratic control.

“The first piece of legislation that just got pushed through the new Congress is a big raft of ethics reform, government reform and leading the way was campaign finance reform. I think that’ll pass the House. I think it will get stuck in the Senate,” Reed observes. ‘But it will at least keep focus on this issue for the next two years and have something on the table that will help put government into the hands of everyday folks instead of just a couple of billionaires.”

Tester is less than optimistic about the prospect of shedding light on dark money.

“I am not hopeful because (Sen.) Mitch McConnell, the majority leader, is ardently against any sort of transparency in money that’s pumped into campaigns. He can stop any bill from coming to the floor,” he states. “But that doesn’t mean you quit fighting. Keep fighting, keep trying to do the right thing because transparency in government is a good thing and funding of campaigns is part of government. So we need to make it more transparent.”