President Donald Trump's trade war with Canada is escalating as the midterm elections approach, threatening Republican efforts to address voters' economic concerns in a year when control of the U.S. Senate hinges on states along the border between the United States and its northern neighbor.

The two-week runoff window since the two Republicans were the top vote-getters in an Aug. 11 primary has been a slugfest.

South Carolina Republicans will return to the polls Tuesday for the fourth time since June to replace the late U.S. Sen. Lindsey Graham on the fall general election ballot.