KEEP Donald Trump off stage and wash this presidential campaign’s mouth out with soap, and American politics is still broken. That is the result that emerged from a controlled experiment in political science conducted on October 4th—more formally known as the first and only vice-presidential debate of 2016.

The debate pitted Hillary Clinton’s running-mate, Senator Tim Kaine of Virginia against Mr Trump’s sidekick, Governor Mike Pence of Indiana. The two men, both sons of the Midwest with a neat line in folksy, aw-shucks modesty, spent quite a lot of their 90-minute clash stressing how much they respected each other’s faith and essential decency, even as they sparred about tax rates, how to fight terrorism and other questions of policy. As the pair bragged, competitively, about their Middle American credentials, Mr Pence took an early start by announcing: “I grew up with a cornfield in my backyard”. Mr Kaine countered that he had worked with Jesuit missionaries in Honduras as a young man. Neither man swanked about the size of his genitals, called anyone fat or declared himself a genius. With Mr Trump and his almost-equally disliked rival Hillary Clinton absent from the debate hall, whole minutes at a time sounded pretty reasonable.

The vice-presidential debate provided the answer to a very specific question: what happens when a centrist Democrat with minimal political baggage is pitted against a fairly conventional, Reagan-quoting Republican from the Christian conservative wing of the party? Many conditions for a successful experiment were met. It seems safe to assume that the encounter, hosted by Longwood University in rural Virginia, will have had millions of Democrats and Republicans nodding along as their party’s nominee spoke. That is because the two hewed closely to positions that, polls show, are seen as no more than common sense by each party’s respective partisans. Mr Pence touted his stern views on abortion, predicted that tax cuts would cause the economy to take off like a rocket and said that American “strength” should be used to counter bullying by Russia’s president, Vladimir Putin, after years of “feckless” foreign policy by the Obama administration. In each instance these were pillars of Republican orthodoxy (and in the case of Russia-bashing, Mr Pence was taking a stance well to the right of his boss Mr Trump, who has quite the geopolitical crush on Mr Putin, praising his “very strong control” of his country). Mr Pence defended his attempt, as governor of Indiana, to halt all resettlement of Syrian refugees in his state on grounds of public safety—a policy which a federal court this month called unconstitutional discrimination on grounds of nationality, but which makes ample political sense in the light of polling by the Pew Research Centre, showing that 74% of Republicans (and a non-negligible 40% of Democrats) call refugees from the Middle East a “major threat” to America’s well-being.

In his opening statement Mr Kaine noted that the debate site, the small town of Farmville, was the scene of an early civil-rights protest by black school pupils, led by a 16-year-old student, Barbara Johns. The tribute was doubtless sincere: his own father-in-law, a Republican governor of Virginia, had an unusually progressive record on racial segregation. As a young man Mr Kaine worked as a civil-rights lawyer. But the political advantages were not hard to spot: black Americans, the young and women are all voter blocks that must turn out in large numbers if the Clinton-Kaine ticket is to win. Not for nothing did Mr Kaine hail Miss Johns for believing that the nation is “stronger together”—a phrase which just happens to be Mrs Clinton’s campaign slogan.

On issue after issue Mr Kaine and Mr Pence represented the mainstream views of their parties, while avoiding the deeply personal attacks that so blight the Clinton-Trump contest. And yet the results from this experiment should give Americans pause. For the debate revealed vanishingly little common ground between the two men—to the point that it is hard to see how a government divided between their two parties would work. And if Mrs Clinton wins the White House in November, she will face a familiar division: Republicans will keep control of the House of Representatives, and could well hold the Senate too.

Press headlines after the debate focused on Mr Pence’s greater fluency and air of authority, on Mr Kaine’s nervousness, and on the theory that Democrats could console themselves that, at least, the Republican on stage spent much of his time being invited to defend Mr Trump’s nastiest insults. Certainly it was striking to see Mr Pence variously deny that he had heard Mr Trump make such remarks, or simply dismiss them. Asked about Mr Trump’s claim that Mexico sends “rapists” across the border, the Indiana governor scoffed: “You've whipped out that Mexican thing again,” before insisting that “criminal aliens” are committing violence and “taking American lives”.

But the debate also revealed something that will matter long after this election: America’s two parties struggle to agree even on a common set of facts about the state of the nation. Squabbling about whether the economy under Mr Obama has been a disaster or a stirring success, Mr Pence retorted that because voters in mostly white, working-class rust-belt cities are flocking to Mr Trump’s populist banner, that is what counts. Or as he put it: “Honestly, Senator, you can roll out the numbers and the sunny side, but I got to tell you, people in Scranton know different. People in Fort Wayne, Indiana, know different.”

For sure, Mr Trump’s demagoguery and Mrs Clinton’s unpopularity go a long way towards explaining this horrible election. But to extend the laboratory analogy rather far, Mr Trump is like a powerful electrical charge, catalysing a reaction already under way. Take Mrs Clinton off stage, and even a generic Democrat as amiable as Mr Kaine struggles to defend the status quo in a time of voter rage. This crisis is structural.