Calls for runner-up Bill Thompson to bow out of the New York Democratic mayoral race grew louder Thursday, as several of his prominent supporters switched allegiances to front-runner Bill de Blasio in hopes of avoiding a runoff that could weaken the winner for the general election. The city’s Democratic Party, desperate to elect its first mayor since 1989, was unable to officially rally around de Blasio, the city’s public advocate, who finished Tuesday’s primary with just above 40 percent of the votes. That’s the threshold that would have clinched his party’s nomination, but election officials said it will be at...