On the eve of the midterms, President Donald Trump’s approval is falling, young voters are energized, and Republicans look poised to lose their House majority. It’s enough to make Democrats nervous, miserable wrecks. Haunted by memories of 2016, liberals around the country are riven with anxiety in the campaign’s homestretch. They’re suspicious of favorable polls and making election night contingency plans in case their worst fears come true. Some report literal nightmares about a Democratic wipeout. “We're kind of just in the bed-wetting phase now," said Democratic pollster John Anzalone, a Hillary Clinton campaign alumnus who spent election night 2016...