A majority of American voters believe Donald Trump is harming the national economy, according to a new poll.

The rising cost of fuel and other essentials since the start of the Iran war remains the biggest challenge for Republicans approaching the midterm election, the NBC survey of 1000 voters revealed.

A majority (55 per cent) of those surveyed said they believe Trump’s policies have damaged the economy. That’s up from 48 per cent in March, before the Iran conflict triggered the price of consumer goods to spike.

But even with voters increasingly unhappy about Trump, the Democrats are no shoo-in to seize control of Congress in November.

The poll showed the Democratic Party hold a five-point lead, way less than the 12-point advantage the party enjoyed at this point before the 2018 election during Trump’s first term.

A truck drives past a Minneapolis fuel station as diesel prices hit a record high in the US, on September 4. (AP Photo/Ellen Schmidt, File)