Pity the folks of magnificent Montana who go to the polls this week to elect a new member of Congress. They must choose between two candidates each from the extreme fringes of their respective parties, proxies for Bernie Sanders and Donald Trump. Did it not occur to anybody that in this most tumultuous of electoral times a moderate might have had greater appeal?

This is the lag in national politics in America. Last November, the country defied the pundits and pollsters by electing a president for whom the term ‘outsider’ is entirely inadequate. The magic elixir had thus been discovered: from there on out, the two main parties were on the hunt for prospective candidates who were as far removed from the political mainstream as possible. Experience and competence be damned, the era of non-politician politicians had arrived.

Thursday’s special election out West was called after Congressman Ryan Zinke, a Republican, was tapped by Trump as Secretary of the Interior. His district was the entire state because there aren’t enough Montanans to warrant more than one seat in the House in Washington.

It is being watched very closely. History suggests that Republican Greg Gianforte, a billionaire former software executive, should easily dispatch his Democrat opponent, country and bluegrass musician Rob Quist. Republicans have held the at-large Montana seat for the last two decades and Trump beat Hillary Clinton in the state by more than 20 points last November.

Gianforte, 56, who ran one TV ad showing him firing a rifle at a computer monitor showing graphics indicating his opponent’s support for gun control, has eagerly attached himself to Trump and his policies, including the Obamacare replacement and the conservative draft budget just relayed by the White House to Congress even though both threaten to bring considerable pain to many of the Montana voters who so enthusiastically embraced the president last year.

It is because he has been so in-harness with Trump that a Gianforte loss - and polls show a tightening contest - would set off loud sirens for Republicans. Democrats would sniff an anti-Trump backlash, boosting hopes of prevailing in other upcoming special elections in Georgia, California and South Carolina. More importantly, it would stoke their boilers ahead of the mid-term elections next year when they will have a chance to take back control of Congress.

Quist, 69, has been buoyed by new infusions of money from donors and enthusiastic support from the left-flank of the national party. Last weekend saw him darting about the state in the company of Bernie Sanders. The Vermont Senator, who calls himself a Democratic socialist, drew huge crowds campaigning in the state last year for the Democratic nomination.

Even more than his rival, Quist has branded himself a populist outsider, though of the left. Until this year, he had no experience at all of public service or running for elected office. If he had a public profile it was as a member of the bluegrass Mission Mountain Wood Band. He has made frequent musical appearances at a large nudist resort in Idaho. He had his own TV ad firing bullets at a television screen playing one of Gianforte’s spots. Uplifting stuff. Quist has suffered other setbacks, including revelations of past debts, unpaid taxes and venereal disease.

What was it that persuaded both parties in Montana that Trump’s victory last November meant a new and fixed template had been set for electoral success across America going forward? Might not the Democrats, in particular, have been better served recognising that what seems like a good idea to voters one year looks altogether less clever the next?

It is what Fred Davis, a veteran Republican political strategist in California who advised the presidential campaigns of John Kasich and John McCain calls the law of the pendulum. Nothing stands still; the silver bullet of one season may be the dud of the following cycle.

“There’s an often-fatal flaw in political thinking that recent-past elections predict the future,” he explains. “So, like now, when a trove of people have been elected with the title “outsider,” that naturally means that to win in the future everyone must be labeled an outsider. Right? Wrong. Politics is one big pendulum, it never stops and never will.”

Davis recalls that Barack Obama was rudely thwacked by the pendulum effect in 2010 when Democrats suffered crushing defeats in mid-term elections. Things swung back for him in time for his 2012 re-election but then along came 2016, a disaster for Democrats.

“What does this tell us about 2018, 2020?” the amicable Davis goes on. “It should tell us that the label ‘outsider’ will not necessarily guarantee a win. That the pendulum is more likely to move from ‘outsiders’ brimming with grand ideas to those who can actually accomplish something. Those with actual, appropriate experience for the job. The pendulum never stops.”

In London the other week, I found myself thinking how Brexit felt so 2016. The pendulum swings in Britain too. The Leave sell-by date had already expired. The tragedy is that while voters regularly have the chance to vote politicians in or out according to their shifting sentiments, with their decision to leave the European Union the people of Britain have pitched themselves into a rut it might take decades to climb out of again.

Here in America, meanwhile, any shift away from populism should offer hope to Democrats for the gains they might make in the 2018 midterms. The more Trump stumbles and the more his inexperience proves to be liability rather than advantage, the harder the pendulum will surely swing. That that hasn’t happened quite yet - his base of support seems largely to be holding up - doesn’t mean that it won’t soon. He has only been president for four months after all.