The defining question in US politics was asked 44 years ago this month. One week before the 1980 presidential election, Ronald Reagan and Jimmy Carter squared up to each other for a televised debate. A former Hollywood actor, Reagan was also proving a lethal Washington aphorist. At the close, he spoke into the camera: “Next Tuesday, all of you will go to the polls. You’ll stand there in the polling place and make a decision.” Watching at home were more than 80 million Americans. “When you make that decision … ask yourself: are you better off than you were four years ago?” Is it easier to buy things, he asked, is unemployment lower?

A few days later, voters gave their answer, handing Reagan a 44-state landslide. Every presidential contest since has been framed in large part by his simple, deadly question. Ask it in the final stretch of this election and you get to the great mystery of why the race remains so close.

Are Americans better off than they were four years ago? Pretty much every mainstream economist would say: you bet. Many go further. “I’ve hesitated to say this at the risk of sounding hyperbolic,” wrote Mark Zandi, the respected chief economist at Moodys, just a few days ago. “But … there is no denying it: this is among the best performing economies in my 35+ years as an economist.” Growth: up. Jobs: up. Wages: rising. The value of your home: up. Share prices: booming. Inflation: falling. Borrowing rates: dropping.

In 2020, Donald Trump warned that his defeat would produce “a depression”. Today, even while Germany and Japan face recession, magazines toast the US economy’s “superstar status”. Yet ask Americans if they feel better off, and many answer: no.

Under Reagan’s law, this election ought to be in the bag for Kamala Harris. As Joe Biden’s number two, she can claim co-authorship for this boom. Instead, she is neck and neck with a convicted criminal (never forget: three weeks after polling day, a judge will decide if Donald Trump should be jailed over the hush money paid to Stormy Daniels). On the economy, Trump regularly polls ahead of Harris. The issue that ought to be winning for her is instead losing.

How come? It ranks among the most consequential questions of our time, yet, however hard they scratch their heads Washington’s finest can’t give a good answer. Many on the centre left paint it as a PR problem: that Biden has failed to claim the credit or that voters are too dumb to realise how good things are. But another suggestion emerges in a new report from a progressive thinktank, the Democracy Collaborative. And its argument should be heeded by Keir Starmer and the European left.

The authors examine much the same economic dashboard as everyone else – growth, jobs, wages – but over a far longer timeframe. Behind each graph lies the implicit question: are you, your family, your community better off than you were not four years ago, but two, three, four decades ago? And for many people the numbers say: no.

Take the biggest one: pay. For teachers, clerical workers, sales reps and the great bulk of US employees, whether white or blue collar, wages have flatlined – not for four or even 20 years – but for most of the past half century. Strip out inflation and average hourly earnings for seven out of 10 US employees have barely risen since Richard Nixon was in the White House.

For the average US employee, and their families and their towns, the economy has kept on tanking whoever wins the White House, whichever judges make it to the supreme court, whether the analysts decree it to be boom or bust.

Biden has spent trillions on boosting the economy and adapting to the climate crisis. He has bolstered unions and intervened in strikes. The graphs show it has had an impact – but it is a tiny uptick at the end of a line that otherwise points remorselessly down. Americans are better off than they were four years ago, it’s just that many were in distress in 2020.

Reagan destroyed their unions, Bill Clinton threw open their trade barriers, George Bush Jr dispatched their kids to fight and die abroad, Barack Obama bailed out Wall Street and Trump ran a glorified protection racket. Only in 2020 did real wages for “production and non-supervisory employees” rise above where they were in 1973. This was not because they were unproductive: the US economy continues to do more with less almost every year. It’s just that most of the gains from that have gone to the top.

“Even if Trump loses, America remains very vulnerable to a far nastier imitation winning next time,” says Joe Guinan, president of the Democracy Collaborative. The only way to see off Trump, JD Vance and the pluto-populists is to make the economy more equal, to give workers more of a stake in the riches they produce.

To see how that plays out, I checked in on Mike Stout. We first talked in a diner in Pittsburgh in 2012, the year Obama won re-election. Mike and his wife, Steffi, had worked in Pennsylvania’s steel industry, with good union pay and pensions. They’d gone to Washington for the first inauguration of Obama, and stood in the freezing January cold. They had hopes.

The Stouts did everything right. Worked hard and saved, and spent $50,000 to get their kids through university. In 2012, their daughter Maura was working in a downtown hotel for $14 an hour, the same as her father had earned in 1978. Even then she doubted whether she and her husband would ever enjoy the same standard of living as her parents.

She’d lost that hotel job during the pandemic, said Mike, and was working from her one-bedroom flat. Her job was chasing people for their debts, even though at $18 an hour she was only just keeping her head above water. Now in her 30s, she’d split from her husband, and Mike thought much of the blame lay in money problems. As for his son, Mike, he was looking after his wife, who has stage 4 cancer, and their kids. Mike has health insurance, which counts as good fortune in the US, but the top-up fees are eye-watering, and now he works two jobs.

“They are teetering on a ledge 60 floors up,” said Stout. “The slightest nuance – a recession or prices going up again – and they’re pushed out of the window.”

Life for the Stouts has been frozen for years. At the root of democratic capitalism is an old promise: tomorrow will be better than today. But that promise was broken long ago for Mike’s family and many of his friends’ households, too. He knew plenty of former steelworkers in this swing state who next month would vote Trump. Sure he was a liar, “but at least he lies to their faces, rather than ignoring them”.

And what about Mike? “Trump or Harris: it’s just one big uni-party,” he said. “It’s Wall Street that runs this country.”

• Aditya Chakrabortty is a Guardian columnist