After almost a year off the platform, Donald Trump returned to X this week and asked his 89 million followers: “Are you better off now than you were when I was president?”

It was a clear echo of the famous quote from Ronald Reagan during his victorious 1980 presidential campaign when he asked: “Are you better off today than you were four years ago?”

This messaging isn’t surprising. It seems like an obvious strategy for Trump to focus on the economy.

That’s because polls consistently suggest it’s the issue American voters care about the most. One such poll conducted by The Economist and YouGov in recent days listed “inflation/prices” and “jobs and the economy” among voters’ top concerns.

Perhaps more importantly, polls also indicate voters are deeply unhappy with the current state of affairs.

That seems like a perfect situation for any presidential challenger.

But in an election that’s been transformed by Kamala Harris taking over from Joe Biden as the Democratic candidate, Trump seems to be struggling to land his simple message on the economy.

It is less than a month since he was on stage at the Republican National Convention looking unbeatable, having survived an assassination attempt and riding high in the opinion polls.

Now, he has lost that lead and seems to have lost his way. Meanwhile, in the opposition corner, Ms Harris is riding a wave of excitement and enthusiasm that he is finding difficult to counter.

The easiest way to burst her bubble would be to remind voters how unhappy they are about high prices and blame her for the inflation that has pushed up the cost of living during the time she has been beside President Biden in the White House.

One of the reasons Trump is failing to land that message is the Harris campaign’s strategy of putting proposals to try to lower the cost of living at the heart of her pitch.

In a speech in North Carolina on Friday, Ms Harris promised to expand child tax credits, help people to purchase their first homes, and to encourage the building of more affordable housing.

She also said she hoped to tackle the persistently high price of food and groceries by banning “price gouging” or excessive corporate profiteering.

"By any measure, our economy is the strongest in the world,” she said. “Many Americans don't yet feel that progress in their daily lives."

Joe Biden and Kamala Harris ought to be able to boast about some very good economic indicators. There is strong growth, record levels of job creation, and this week the inflation rate fell below 3% for the first time during Mr Biden’s presidency.

But because prices are still high, voters don’t feel any better off. Voters don’t care about the rate of inflation – they care about the level of prices.

“A central banker wants inflation to get back to target. A shopper wants his or her old price back," Jared Bernstein, the chair of President Biden’s Council of Economic Advisers, said in a July speech.

When it comes to the economy, “the vibes are off”.

“Vibes matter,” Mr Bernstein said.

So will the bad economic vibes hurt the Harris campaign?

That is what I asked voters over lunch at a crab shack on the Chesapeake Bay in Maryland.

Jeff Tester, who works in a nearby marina, said high prices are really hurting him.

“I get paid by the hour. I get up to go to work every day. I think you have to do that to get the American dream,” he said. “But I just know it's getting harder.”

And he is very clear about who he sees as responsible. “I blame the Democrats. I believe their policies are hurting the working man,” he said.

Every diner I met complained about inflation, but not everyone held Mr Biden or Ms Harris responsible.

Dan Nardo, a retired boat broker, said he believed the pandemic, oil prices, foreign wars and supply chain issues have more to do with price rises than the US president.

His friend Randy Turk, a retired lawyer, told me that he felt a new administration is likely to follow a similar path to try to reduce inflation, regardless of who wins.

“It's not like a different president can really make that much of a difference,” he said.

Ms Harris struggled for prominence and media coverage during most of her time as vice-president. Previously that was seen as a weakness. But if it means she can emerge untainted by “Bidenomics”, it could be one of her greatest strengths.

Ruth Igielnik, polling editor at the New York Times, says the latest data she has collected suggests “voters very much tied their negative feelings about the economy to Joe Biden”.

Talking to me on the BBC’s Americast podcast, she explained that in her polling Trump is still favoured on the economy, but where he once had an 18-point lead over Mr Biden he now leads Ms Harris by only about 8 points.

“That makes me think voters aren’t necessarily attaching their feelings about the economy to her,” she said.

A separate poll this week conducted for the Financial Times and the University of Michigan Ross School of Business indicated Ms Harris holds a narrow lead over Trump on who Americans trust to handle the economy.

No wonder Republicans are publicly begging Trump to focus on the issues, the economy in particular, and stop launching personal attacks against Ms Harris.

In a speech this week, Trump told supporters he was going to talk about the economy but struggled to stay on topic.

“They say it’s the most important subject,” he said, "they" referring to his advisers and strategists who believe this is his strongest line of attack.

“I’m not sure it is. But they say it’s the most important,” he added, before going on to list immigration, crime and the way Ms Harris laughs as top issues. You could practically hear his campaign managers pulling their hair out.

“Voters don’t care about personalities or who is drawing larger crowd sizes,” said Matt Terrill, former chief of staff for Marco Rubio’s presidential campaign.

“Independent, undecided, swing voters in key states care about the economy and inflation so just focus on those core issues,” he said.

“Stay focused on talking about how you are going to make the lives of Americans better over the next four years.”

It was back in 1992 that the Democrat Jim Carville coined the slogan “It’s the economy, stupid” while he was working on Bill Clinton’s presidential campaign.

It’s advice that every campaign since has clung to. But Trump, this time around, seems to be finding it uncommonly difficult to stick with.

It ought to be a winner for him. After all, according to the Financial Times poll, in answer to his question “Are you better off now than you were when I was president?” only 19% of voters say they are.