White working class is shrinking. It still may decide 2020 election.

Almost every morning, half a dozen men meet for coffee at Max’s Deli, a diner that shares a wall with an auto supply shop, to talk politics. Some live “up the line” or “down the line” – a reference to the historic Scranton railway – but all have lived in the area their entire lives. And come Nov. 3, all are planning to vote for President Donald Trump.

“I’ve never really been political up until this last election,” says Bob Duffy, an Air Force veteran and retired mailman. He expresses...