A tight three-way primary race is underway in West Virginia for the Republican nomination for Senate.

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President Trump on Monday intervened and pleaded with voters to oppose one of the candidates, Don Blankenship, a former coal mining executive who spent a year in prison for his role in a fatal mining explosion.

The winner of the primary would compete against the incumbent, Senator Joe Manchin III, who is seen as a deeply vulnerable Democrat in a state that Mr. Trump won by 68 percent in 2016.

West Virginia, historically friendly ground for Democrats, has shifted sharply to the right in recent years in part because of anger at the Obama administration and what many there saw as its hostility to the coal industry.

Here’s what to watch for in West Virginia’s primary elections.