WASHINGTON — Rarely has a municipal election in a midsize city ignited such a fierce national debate over what defines a Democrat. But the election this spring for mayor of Omaha is pitting abortion rights activists against economic populists and threatening the party’s unity as it regroups to confront President Trump. Part of the twist: The populist side’s standard-bearer, Senator Bernie Sanders of Vermont, is not even a Democrat. (snip) Mr. Sanders and the new leadership of the Democratic National Committee touched a party sore spot this week when they took their “Unity Tour” to Omaha to rally for a...