Before last month’s special congressional election in Pennsylvania, U.S. President Donald Trump, who ridiculed his rivals in the 2016 election as “Liddle Marco,” “Lyin’ Ted,” and “Crooked Hillary,” tried it again, tagging Democratic candidate Conor Lamb as “Lamb the Sham.” This time, the tactic failed. But it illustrated a penchant for often brutally effective nicknames that Democrats are bracing for Trump to employ when he campaigns for Republicans in November’s congressional elections. All 435 members of the House and a third of the 100-member Senate will be up for grabs. A Democratic takeover of one or both chambers could derail...