Of all the places Democrats could hunker down for a long fight in their epic 50-state scramble for the presidential nomination, Pennsylvania is perhaps the most illuminating. Politically speaking, when Pennsylvania gets the sniffles, America braces for a fever.
Just ask the first President Bush, whose approval ratings were the very picture of political health in the spring of 1991. Then a freak accident killed Pennsylvania's GOP Senator John Heinz, and in the special election to replace him, a liberal Democrat named Harris Wofford diagnosed an unease in the electorate about endangered jobs and affordable health care. Hammering at these issues,...