SIOUX CITY, Iowa — To describe what he would do as president in 2021, Steve Bullock harks back to the Gilded Age of the late 1800s, when the wealthy copper mining barons of Montana wielded political influence across the state.

To rein in the copper kings, the state banned corporate spending in elections in 1912. “Served us well,” the Montana governor said wistfully to a crowd of Democratic caucusgoers gathered at a local bar here, “until Citizens United came down.”

For the next three minutes, Bullock ranted against the 2010 Supreme Court case that opened the door to the modern era of big-money politics, bemoaning the looming “threat of expenditures” by corporations and the influence of “dark money,” from county supervisor elections to his own race as governor.

If this feels arcane, well, it is. Yet, at the end of the three minutes, the Democratic activists gathered here applauded, as one voter yelled out: “Hear, hear.”

Money in politics is the signature campaign issue for Bullock, the 22nd to join the 25-candidate race for the Democratic presidential nomination. To Bullock, the most urgent issues on voters’ minds — income inequality, climate change, taxes, collective bargaining rights and more — stem from the lasting influence of moneyed interests.

Bullock has been promoting this message across Iowa since joining the race in mid-May, making his case even as he failed to clear a single digit in enough polls to qualify for the first round of Democratic debates last month.

In the past week, Bullock made his sixth campaign trip to the first-in-the-nation caucus state of Iowa. And he is poised to take his message to a national audience at the upcoming debate in late July, having garnered the necessary support to qualify for the stage this time.

There are signs his message may be resonating: He pulled in $2 million in donations since joining the race on May 14, his campaign said, a decent haul for a relatively unknown politician outside his state.

But he faces several challenges as he attempts to make his leap from national obscurity to the top of the party’s presidential ticket.

Bullock boasts a strong record fighting the influence of wealthy donors. As attorney general, Bullock defended the 1912 ban, which was overturned after the Supreme Court’s decision in Citizens United v. Federal Election Commission. And he is suing the Trump administration over a regulation that allows certain political nonprofit groups to obfuscate the identities of their donors.

As governor, he signed an executive order to require all state government contractors to disclose their political spending, something he says he would do as president.

The anti-big-money message is one that can elicit a visceral reaction among voters and cultivate a loyal following — for example, President Trump’s “drain the swamp” call in 2016 and Sen. Bernie Sanders’s rant about the wrongs of the “millionaires and billionaires.”

But Bullock has yet to find that pithy, relatable message, his campaign acknowledges.

According to the western Iowa voters who recently listened intently through Bullock’s stump speech — the 10th or 11th candidate to swing through, by their count — money in politics ranked somewhere between “inside baseball” and “a major concern.”

Bullock is one of many talking about money in politics in the saturated campaign.

“I think Kirsten Gillibrand is also kind of talking about campaign finance,” said Scott Cordell, a 62-year-old retired science teacher, after listening to Bullock speak at a restaurant in Council Bluffs, Iowa. “Many of the Democratic candidates are very similar in policy.”

Bullock is the only Democratic presidential candidate to win a statewide office in a place that President Trump carried in 2016, and is trying to carve a centrist path toward the nomination. He hopes to appeal to independents and moderate Republicans seeking an alternative to Trump.

He answers with ease when voters ask about his strategies for working with a Republican-controlled legislature, increasing health-care access and improving the state’s education system. When asked about gun control, Bullock talks about his experience as a hunter, as a father of a son who came home from school to describe an active-shooter drill, and as a governor who ordered state flags at half-staff multiple times for shooting victims.

This has suited him well in Montana, where voters elected him to his second term as governor while voting for Trump by 20 points. But in the crowded primary, he is struggling to stand out. He is among the presidential hopefuls that the party’s prominent donors and strategists are trying to convince to drop out and run for Senate instead.

Bullock, 53, is uniquely well-known in at least one circle of Americans: those steeped in the wonky, and at times esoteric, world of campaign finance and the regulation of the giving that shapes national politics.

In this world, his record fighting what is derided as dark money — the unlimited money flowing from undisclosed donors to try to influence elections — in Montana was so influential that it was featured in a movie, aptly titled “Dark Money.” It premiered at the Sundance Film Festival in January of last year and was shown in select indie theaters before airing on PBS.

“This fight over money in politics is kind of baked into Montana politics in a more public way than a lot of other states,” said Meredith McGehee, a longtime advocate of greater regulation of money in politics.

“Other candidates have talked about it, but [Bullock], while at the state level, took this on as one of his signature issues,” said McGehee, the executive director of advocacy group Issue One.

On the presidential campaign trail, political money is the through-line with which he connects the issues that he believes voters care about.

“Washington is really captured by the money,” he told voters near Des Moines.

Those in the audience overwhelmingly said they agreed with him. But several voters said they wanted to hear more, beyond the influence of money.

Citizens United “is one of the most horrific decisions ever made, I think,” said Lynne Gentry, 71, a retired teacher from Rockwell City. But on her top issue of immigration, she found Bullock’s answer lackluster. “He agreed that, yes, it’s important. But, why?”

Patti Naylor, 63, an organic farmer from Churdan, Iowa, attended a coffee shop event hoping to learn about Bullock’s stance on the impact of the trade war on soybean farmers, her top priority. She took notes as Bullock answered her question, then when he finished speaking, she let out a sigh and shook her head.

“He did not answer my question,” she said during an interview afterward. “My note to myself was that he’s really good at not answering the question directly. . . . Health care, what’s he going to do about that? Trade, it sounds like trade isn’t going to change at all.”

Naylor said money in politics was “a huge” issue. “But you can’t just run on one issue,” she added. “It’s not just about the dark money. You have to have the policies that you’re going to change to make it a better world.”