United States (U.S.) President Donald Trump's strongest case for re-election is arguably the economy —he says so himself. But that argument might be slipping with voters, according to the latest Quinnipiac University poll.

For the first time since Mr. Trump won the presidency in 2016, more registered voters say the country's economy is getting worse than say it's getting better.

Voters still think the economy is good, but of those polled, 37% say the economy is getting worse, compared with 31% who say it's getting better and 30% who say it's staying the same. Just two months earlier in June, 23% of voters Quinnipiac surveyed said the economy was getting worse.

“As trade tensions with China dominate the headlines, confidence in the economy is slipping," Quinnipiac University Polling analyst Mary Snow said in a statement.

“The number of people who think the economy is getting worse rose by double digits since June. And roughly four in 10 voters blame the President's policies, saying they are hurting the economy, the highest level since Trump took office.”

A CBS News poll released this week, however, indicated Americans are generally optimistic about the economy in the future. The poll found that 38% of Americans — who are not necessarily registered voters — 35% are pessimistic and 27% are not sure.

Volatile stalks, an inverted yield curve and uncertainties over the president's trade war with China have a number of economists concerned a recession could hit in the next two years.

The president typically lists economic gains as his top accomplishment, and the state of the economy is highlighted frequently in administration and campaign talking points. But if voters don't see the world the same way, that message could be undermined.

“I won the election, the markets went up thousands of points, things started happening.”

“If, for some reason, I were not to have won the election, these markets would have crashed. That will happen even more so in 2020.

“You have no choice but to vote for me, because your 401(k), everything is going to be down the tubes.”

If there is no recession, and voters continue to think the economy is good, even if people “think the economy is heading in the wrong direction, that's probably good enough for the president's prospects, said GOP strategist Ford O'Connell.

“Since FDR, every incumbent president who has avoided a recession has been reelected,” O'Connell told CBS News.

Most economist don't think there will be a recession before next year's presidential election, a recession may be likely in 2021.

Democrats, O'Connell said, are going “full-court press” to convince voters that the economy is heading for a recession.

The White House and Trump campaign will continue emphasising the message that the economy is strong —and would worsen exponentially under a Democrat, O'Connell said. (CBS News)

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