Democrats begin the Donald Trump presidency in sad shape. They lack a clear power base, they’ve got no distinct national leader, and party brokers are searching for a formula to counter the new Republican-dominated government and figure out how to win again. It’s a curious and dispiriting position for a party that has led the national popular vote six out of the past seven presidential elections. Yet Hillary Clinton lost the Electoral College count, while Republicans maintained their largest House majority since 1928 and kept control of the Senate — with 2018 advantages that offer the potential of a Senate...