CARROLL, Iowa—Rumbling across western Iowa by bus, Joe Biden is seeking to steady his campaign by appealing directly to rural voters he says have been hurt by President Trump’s trade war with China.

The former vice president is traveling the state on an 8-day, 18-county bus tour, his longest stretch of campaigning in Iowa, as polls during the past month have shown him falling behind South Bend, Ind., Mayor Pete Buttigieg and wrapped in a tight contest against Sens. Elizabeth Warren of Massachusetts and Bernie Sanders of Vermont.

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