Democrats hoping to regain control of the Senate in this year’s midterm elections face a tough task: They must flip four seats controlled by Republicans, on a map that offers few obvious opportunities. Midterm elections are traditionally an uphill battle for the party in the White House, and some of President Trump’s unpopular policies could hinder his party’s efforts to maintain a majority in the Senate and continue advancing his agenda. But Democrats control only 47 seats, compared with 53 for Republicans, and they also have to defend vulnerable seats of their own. Still, Democrats in recent months have had...