WASHINGTON -- Sen. Bob Casey, a Democrat from Pennsylvania, lives along so many fault lines of American politics that he is especially sensitive to Trump tremors, which he fears could become an earthquake by November. “I’m concerned,” he said. “Beating Donald Trump won’t be as easy as it might look.” Casey is a pro-life, pro-gun Roman Catholic in a state that Democratic consultant James Carville once described as “Pittsburgh and Philadelphia with Alabama in between.” He is also an old-school Democrat and a new-school one: He's pro-union and wary of global trade; he defends Social Security, Medicare, Obamacare and same-sex...