A key measure of manufacturing activity in the United States has indicated a contraction for the first time in nearly a decade, adding to worries for the economy.

The U.S. manufacturing PMI, or purchasing managers’ index, fell below 50 for the first time since September, 2009, meaning that the sector is shrinking rather than growing.

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The index, compiled by financial information company IHS Markit, tracks a slew of factors such as output, new orders, prices and employment to capture a snapshot of the industry. The index settled to 50 in July, and fell to 49.9 in August.

The drop is the latest worrisome sign for the manufacturing sector after a period of growth early in Donald TrumpDonald John TrumpSarah Huckabee Sanders becomes Fox News contributor The US-Iranian scuffle over a ship is a sideshow to events in the Gulf South Korea: US, North Korea to resume nuclear talks ‘soon’ MORE‘s presidency. Federal Reserve data showed manufacturing output contracting for the past two quarters, meeting a commonly used definition for a recession.

The news comes alongside heightened concerns of a broader economic slowdown or recession. A common bond market signal known as an inverted yield curve on Thursday flashed for the third time since last Wednesday.

If the trend continues, it could spell trouble for Trump’s reelection campaign. Manufacturing plays an outsized role in Pennsylvania, Michigan and Wisconsin, the three traditionally blue states he narrowly flipped in 2016 to win the White House.

Trump took to Twitter Wednesday morning, arguing that “The Economy is doing really well,” and pressuring the Federal Reserve to further cut interest rates.