Undetected corporate and foreign influence in elections is only multiplying.

In 2010, the US Supreme Court declared that pumping corporate cash into elections is a form of free speech. As Republican Mitt Romney put it later, "Corporations are people, my friend."

We have been witnessing the consequences of the high court's Citizens United ruling ever since. But because so many of those consequences benefit Republican politicians, there can be a tendency to view opposition to Citizens United as primarily a leftist preoccupation.

What's brilliant about Dark Money, a powerful documentary directed by Montana native Kimberly Reed, is how it proves the falsity of this notion by telling the story of Republican state lawmakers who found themselves in the crosshairs of a very effective dark-money blitz.

The point of the blitz was to purify the Montana State Legislature's Republican caucus. It did this by using dark money to carpet-bomb Republican primary elections with mailers and other electioneering materials aimed at kicking out conservatives who weren't viewed as conservative enough. Those impure conservatives would then be replaced by newly elected Republicans who were, in effect, wholly owned subsidiaries of the dark-money groups that had defeated their opponents.

The blitz worked, and it is quite something to watch shell-shocked Republican politicians speaking directly to the camera—while driving pickup trucks, while sitting in no-frills rural meeting rooms—and arguing passionately that no, money should not be equated with speech, and no, we should not allow out-of-state corporations, or any corporations for that matter, to meddle invisibly in our elections.

It all makes the problem crystal clear: Corporate power is corrupting American democracy using massive contributions and complex voter-manipulation schemes at levels we've never seen before—levels that, to this day, we barely comprehend. It's a dynamic that hurts community-accountable Republicans and Democrats alike while helping only cosseted plutocrats.

The film also makes the solutions crystal clear: daylight and regulation. It follows dogged journalists who endure downsizings and rough indignities (one ends up living and working in his pickup truck) in order to pursue the leaked documents and money trails that will eventually allow Montanans to understand what's happened to their democratic process. In response, state lawmakers—with a key assist from a Montana Republican who was targeted by dark money—pass a tough new transparency law aimed at curbing dark money's influence.

Trump won Montana by 20 points, and pretty much no one in this film comes off as anything close to urbane or polished (except maybe the slick suit running a fake "public meeting" in Butte that's funded by out-of-state interests working against Medicare expansion). But because of the state's unique history of standing up to unabashedly greedy mining barons in the early 20th century, and now the Citizens United–cloaked corporate barons of the early 21st century, Trump-loving Montana has emerged as a national model for how to begin to take back democracy.

It won't be easy. Dark Money makes clear that as social media and other parts of the internet continue to merge with the consequences of Citizens United, the opportunities for undetected corporate and foreign influence in elections will only multiply.

But as the documentary also makes clear, the alternative is well worth fighting against: a grim dystopia in which oligarchs own every step of the "democratic" process, so that government becomes merely another corporate product.