Mark Penn served as senior adviser and pollster to Bill and Hillary Clinton from 1995 to 2008. He is president of the Stagwell Group. Last week's Democratic mini-wave has party members elated, and there is no question that the party is poised for gains in 2018 and perhaps 2020. Capitalizing on this opportunity means first understanding that Nov. 7 showed that moderate candidates offer the best chance for a bigger victory in 2018 and beyond. And to attract these candidates, Democrats need to fix their party...