“OH MY God, I’d lay odds that’s pet-coke,” an appalled Canadian can be heard saying, over amateur video of a wall of black dust, hundreds of feet high, rising from the American side of the Detroit river. He is talking about petroleum coke, a sticky, gritty by-product of oil refining, which—in that video, shot during a 2013 storm—is being blown from a giant mound in industrial Detroit and across to the leafier Canadian city of Windsor.

Lexington would lay odds that TV viewers in Detroit and the surrounding state of Michigan will soon know that video well. That is because the flying filth belonged to a firm controlled by David and Charles Koch (pronounced “coke”), industrialists with a passion for conservative politics. Seven months before the mid-term elections, Americans for Prosperity (AFP), a free-market group backed by the Koch brothers, has already spent millions of dollars on TV advertisements pounding Gary Peters, a Democratic congressman seeking to become Michigan’s next senator. The most potent feature Julie Boonstra, a Michigan cancer patient obliged to change insurance plans by the Obamacare health law. In them Mrs Boonstra talks about her cancelled policy and her “unaffordable” new plan. Addressing Mr Peters directly, she tells him that by voting for Obamacare, he has jeopardised her health.

Similar AFP ads have rained down on vulnerable Democrats in such states as North Carolina, Louisiana, New Hampshire, Arkansas and Alaska, where conservative voters are numerous. The ads are brutal masterpieces. Sometimes using actors and sometimes real voters, often filmed in bleached-out colours and backed by a mournful piano, they depict lives being ruined by the new law. Anguished women open letters bearing bad news from their insurance providers. After opening his letter, a rural father gazes, stricken, at the young daughter he is powerless to protect.

For too long, says Tim Phillips, AFP’s president, “economic freedom” conservatives wooed voters with arguments full of numbers. AFP’s new strategy involves “telling stories” about individuals. The group plans to keep the focus on Obamacare all the way to the November elections, as a cautionary tale. AFP wants to “drive home how big-government programmes disrupt lives and hurt rather than help people.” That way, ventures Mr Phillips, when the Left offers another government plan, perhaps “something on global warming”, Americans will be more sceptical.

Fact-checkers employed by newspapers and websites raise quibbles. The AFP spots exaggerate the downsides of Obamacare, they argue, making it sound as though millions will be left without any cover. Yet most cancelled plans will be replaced, some by policies that will offer comparable benefits more cheaply—as fact-checkers suggest will be the case with Mrs Boonstra’s new plan (though she disputes this). Alas, elections are not won by quibbling over facts. As Representative Peters concedes: “The number of people who see a fact-check is always smaller than the number who see the ads.”

To fight back, Democrats are attacking the Kochs themselves. Mr Peters—running to fill a Senate seat held for 36 years by Carl Levin, a Democrat—currently represents Michigan’s 14th district in Congress. That includes the Detroit waterfront which, for much of 2013, found itself playing host to a large heap of pet-coke (which is widely traded as a fuel and industrial ingredient). Mr Peters describes complaints about “fugitive dust” blowing into homes, and about pollution running off into the Great Lakes. After some time the pet-coke’s owner was found to be a firm with the Bond-villainesque name of Koch Carbon—though its sloppy storage was the work of an unrelated company. Mr Peters helped locals campaign to have the pet-coke removed from Detroit. On March 21st he was due to revisit the site to receive an election endorsement from the League of Conservation Voters, for this and other eco-activism.

The Kochs claim to worry about the health of Michigan residents, Mr Peters says, but they allowed pet-coke to be dumped on the state. He talks of making the Koch brothers’ agenda “real” to voters, citing the Canadian video as useful evidence. “You had a pile that was four or five storeys high and a city block long. All you have to do is to show dust clouds blowing off the pet-coke pile into the air,” he suggests. In Alaska, Senator Mark Begich is running a TV ad denouncing the “billionaire Koch brothers” for closing a refinery in his state. Never one for understatement, the Democratic majority leader in the Senate, Harry Reid, has called the Kochs “un-American”. The Democratic Senatorial Campaign Committee has launched a website describing the Republicans as “addicted to Koch” (and explaining that the Kochs’ “ideal America” is a land of “dirty air” and “tax breaks for billionaires”).

“I’d like to buy the world”—a Koch

Will Koch-bashing work? Many Republicans doubt it, for the simple reason that few voters have heard of the brothers. They dismiss the tactic as a way to scare up donations and motivate core supporters. Some draw parallels with 2006, when conservatives raged about George Soros and other big donors funding scurrilous attack ads from the Left. The rage was in vain: voters still walloped Republicans in a referendum on George Bush and Iraq.

However, Koch-bashing need not hit its purported targets to work (just as attack ads can do the job, even after fact-checkers debunk them). A Democratic Party bigwig cites the 2012 election, when Mitt Romney was denounced as a wicked capitalist who loved to ship jobs to China. Republicans hit back with details of Mr Romney’s successful business record. The chumps! People may not know who David and Charles Koch are “specifically”, says the Democrat. But middle-class voters do believe that the economy is tilted towards the super-rich. Tycoon-bashing is about telling voters that they are right, and that if the Republican agenda looks horrible to them, that is because it is bought and paid for by billionaires. Stories, not arguments.