When he isn't raving about how the deep state is conspiring against him, Donald Trump loves to boast about the economy, claiming to have achieved unprecedented things. As it happens, none of his claims are true. While both GDP and employment have registered solid growth, the Trump economy simply seems to have continued a long expansion that began under former president Barack Obama. In fact, someone who looked only at the past 10 years of data would never guess that an election had taken place.

But now it's starting to look as if Trump really will achieve something unique: He may well be the first president of modern times to preside over a slump that can be directly attributed to his own policies, rather than bad luck.

There has always been a deep unfairness about the relationship between economics and politics: Presidents get both credit and blame for events that usually have little to do with their actions. Jimmy Carter didn't cause the stagflation that put Ronald Reagan in the White House; George Bush didn't cause the economic weakness that elected Bill Clinton; even George W Bush bears at most tangential responsibility for the 2008 financial crisis.

More recently, the "mini-recession" of 2015-16, a slump in manufacturing that may have tipped the scale to Trump, was caused mainly by a plunge in energy prices rather than any of Barack Obama's policies.