Billionaire industrialist David Koch was the subject of attacks by Sen. Harry Reid and other Democrats during the midterm elections. (Carlo Allegri/Reuters)

One clear lesson emerged from last week’s midterm elections: Running against big money in politics is hard to do.

Democrats and their allies made the topic one of their central lines of attack this year, featuring the billionaire industrialists Charles and David Koch in nearly 100 different political spots that ran in states from Alaska to Florida. But the issue failed to gain traction, and most of those Democrats lost.

The difficulty they encountered in transforming the public’s disgust with rich donors into political action speaks to how hard it is to move voters who view both parties as captives of wealthy patrons.

Even as Senate Majority Leader Harry M. Reid (D-Nev.) repeatedly railed against the Kochs in speeches on the Senate floor, billionaire hedge-fund manager Tom Steyer commanded attention this year for the tens of millions he poured into a super PAC backing Democratic candidates.

“It’s very understandable for voters to feel like there’s a pox on both houses,” said Nick Penniman, executive director of Issue One, a bipartisan group working to reduce the influence of wealthy interests in politics.

This year’s midterm races have seen a record amount of outside spending.

The 2014 campaign should have presented a ripe environment to push a message about money in politics. Super PACs and other groups reported spending more than $550 million on congressional races, a record for midterm elections. Voters complained bitterly about waves of negative ads that were backed by out-of-state outfits.

Across the country, candidates sparred over the influence of their rich contributors, while new reform-minded super PACs sought to make campaign money a potent political issue.

But the argument proved to have its limits.

Mayday PAC, which launched with much fanfare as the “super PAC to end super PACs,” failed to play a decisive role in any race.

Some Democrats argue that the anti-Koch message contributed to their victories in Senate races in New Hampshire and Michigan. But it did not save incumbents in states such as North Carolina, Iowa, Colorado and Arkansas, all of whom highlighted the intense spending against them by the Koch-backed political network.

The answer, some party strategists think, is not to abandon the anti-Koch message but to amplify it.

“The way I view it is that we were just getting started,” said David Brock, founder of American Bridge, the independent pro-Democratic research operation.

Brock’s group plans to dig even deeper into the Kochs as part of an effort to tie them to the incoming class of congressional Republicans, a theme it will then carry into the 2016 presidential race.

“The Kochs themselves were put on the defensive for the first time, and a number of their candidates were put on the defensive when these issues were raised,” he said. “To me, that’s a sign that we’re on to something.”

Top officials in the Koch political operation say Tuesday’s dismal showing for Democrats proved that the left’s strategy was flawed.

“While Harry Reid and Senate Democrats spent their time attacking job creators who spoke out against their failed policies, we focused on the issues voters cared about,” said James Davis, spokesman for Freedom Partners Action Fund, a super PAC financed by the Kochs and other donors. “Americans showed they are tired of the political games — they want solutions.”

Some activists seeking to curb the political power of wealthy interests also see the anti-Koch strategy as problematic.

“I think any time you vilify a single family, you create a too-narrow perspective on the problem in a way that inhibits the bigger conversation that we need to be having,” Penniman said. “It’s not about a single family. It’s about a core dysfunction in the American experiment.”

In Kentucky’s Senate campaign this year, liberal groups sought to cast Senate Minority Leader Mitch McConnell as beholden to his campaign donors. The Republican fired back by attacking rich Hollywood figures and other liberal contributors who supported Democratic challenger Alison Lundergan Grimes.

His response proved effective: while 46 percent of Kentucky voters named McConnell as the candidate closest to wealthy donors, 39 percent said it was Grimes, according to an election eve poll conducted by Every Voice Action, a super PAC that seeks to give small donors greater influence in campaigns.

“It’s not enough to just beat up on your opponent, because now we’ve seen evidence in this cycle that your opponent can just beat up on you,” said David Donnelly, the group’s president. “You have to have a real policy agenda and you have to point out where your opponent’s solutions fall short.”

Groups on the left tried myriad approaches to the money-in-politics issue this year.

One of the most high-profile efforts was Mayday PAC, a super PAC started by Harvard Law School professor Lawrence Lessig that spent more than $10 million going after candidates opposed to measures that would lessen the impact of wealthy donors.

In the end, the group could not point to a race in which it turned an election. But Lessig maintains it still had influence, noting that Mayday’s late campaign against Rep. Fred Upton forced the powerful Michigan Republican to plow millions into what most expected was going to be an easy reelection.

“That was a tax on Fred Upton, and other people should look at this and think, ‘Do we want to be in the position to be taxed like Fred Upton if we’re on the wrong side of this issue?’ ” he said.

Most of the messaging about money was focused on the Kochs, a campaign that Reid launched in a speech on the Senate floor. The attack was echoed throughout the year not only by the party’s campaign committees but also by independent groups such as Next­Gen Climate Action, a super PAC largely financed by Steyer.

The Steyer group spent more than $62 million on congressional races, losing the bulk of the campaigns it played in. But Chris Lehane, the group’s chief strategist, argued that the success of Democratic Senate candidate Gary Peters in Michigan, where NextGen sought to tie the Kochs to local pollution issues, “provides a paradigm for the future.”

The take-away for some Democratic strategists this year was that making an issue out of wealthy donors can work.

“A message completely reliant on who is funding candidate X is not going to work on its own, but I think it can be quite effective as part of a larger framework to show how a candidate is out of touch with the people they represent,” said Ali Lapp, executive director of the Democratic super PAC House Majority PAC.

Pollster Geoff Garin, who did work for the super PAC Senate Majority PAC and other Democratic campaigns this year, said he saw evidence that voters were swayed by the anti-Koch message when the ads were at their most intense over the summer.

In one poll of voters in 10 states conducted in early August, 62 percent said they would have a less favorable impression of the Republican Senate candidates if they heard that the Koch brothers were backing them with millions of dollars, Garin said.

“Wherever and whenever those ads ran, they had a breakthrough quality that wasn’t true of other negative advertising,” he said.

The problem, he thinks, is that Democrats did not keep up the line of attack through Election Day.

“If anything, we let up on the Koch brothers too soon,” Garin said.

Some party strategists think the new Congress will provide them with ample opportunities to draw more connections between new GOP lawmakers and their wealthy backers. And they expect the anti-Koch message to continue to dominate in 2016.

There were moments on the campaign trail this fall when Hillary Rodham Clinton, the expected Democratic presidential front-runner, picked up the theme.

At an October rally for Sen. Kay Hagan (D-N.C.), Clinton denounced the “onslaught of out-of-state money and negativity” leveled against the incumbent, urging her supporters to show “that no matter how much money has flooded into this state, North Carolina is not for sale.” Hagan lost on Tuesday.

Bill Clinton went even further on the stump. During a swing through Michigan, the former president singled out the Koch brothers, warning: “They’re spending so much money, what they want after this is a Congress full of Koch pets.”

Alice Crites contributed to this report.