DES MOINES, Iowa—As President Trump maps out his re-election bid, farmers in this battleground state are backing him even with the U.S. Farm Belt bracing for deeper pain from his trade fight with China.

In and around the livestock barns, agriculture building and an antique-tractor collection at the Iowa State Fair in recent days, farmers almost universally expressed support for the president and pledged to vote for him in 2020.

“He’s doing a good job and trying to make sure we’re treated fairly,” said Kevin Prevo, a fifth-generation farmer who raises corn, soybeans, cattle and hogs on about 1,400 acres near Bloomfield, Iowa. Mr. Prevo showed zero uncertainty when asked whether he would vote for Mr. Trump again in 2020. “You bet,” he said.

The battle between the world’s two biggest economies is weighing especially hard on rural America after China said earlier this month that it would suspend all imports of U.S. agricultural goods. The U.S. on Tuesday said it would delay tariffs on some Chinese consumer goods, and Mr. Trump expressed hope that China would commit to some agriculture purchases from America, but it was unclear whether any such pledge was forthcoming.

In the wake of agricultural consolidation in recent decades, farmers aren’t nearly as large a group as they once were. But in heavily rural states like Iowa, which Mr. Trump won by almost 10 percentage points in 2016, they could still be an important voting bloc in 2020.