Six years ago, as a bitter winter gripped the Upper Midwest, Wisconsin Democrats mobilized for a major political protest. Demonstrators packed the streets of Madison tighter than a playoff game at Lambeau Field. They descended upon the Capitol in the tens of thousands to oppose Republican Gov. Scott Walker’s Act 10, which would curtail the influence of the state’s powerful public-employee unions.

Some thought those snowy protests would launch a Democratic surge in Wisconsin. Instead they appear to have marked the beginning...