WASHINGTON (Reuters) - President Donald Trump's approval ratings plummeted in a Washington Post / ABC poll published on Tuesday, highlighting growing concern among Americans about the country's economic prospects.

The good state of the US economy is one of the main themes of the Trump campaign, which will run for a second four-year term by the end of 2020.

"Our economy is strong, our country is strong, we have never been in a better position," he said in a video posted on Twitter over the weekend.

The latest poll by the Washington Post and "ABC" showed that the margin of Trump's popularity among Americans of voting age was 38 compared to 44% in June.

Though 56 percent of Americans think the economy is "excellent" or "good", the figure is down from 65 percent a year ago.

On the other hand, 6 out of 10 Americans considered it “very likely” or “likely” to contract next year.

Americans' view of the way Trump conducts trade negotiations with China has also been harsh, with only 35 percent of those polled saying they approve of his performance and 56 percent opposing it.

The worrying figure for the US president is that 43 percent of those polled believe his trade and economic policies have increased the chances of deflation next year (compared to 16 percent who think they have removed the specter of deflation).

Six out of 10 Americans, on the other hand, expressed concern that a trade war with Beijing could lead to higher prices for consumer products for their families.

The Trump administration has reiterated for several weeks that concern about the state of the economy is unfounded.

US Treasury Secretary Steve Mnuchin said on Monday: "I do not see the possibility of any contraction at all," stressing that Trump's trade war with China "did not affect" the economy of the world's largest power.

The telephone poll was conducted between September 2 and 5 on a representative sample of 1,003 people.