There aren’t many places where low turnout among young and nonwhite voters hurts Democrats more than in North Carolina, a state newly competitive in presidential elections because of rapid generational and demographic changes over the last decade.

Yet nowhere did a Democratic senator lose a closer race in the midterms last month. Kay Hagan, the incumbent, lost by 1.6 percentage points, even though most polls had showed her ahead, and even though the Democratic Senatorial Campaign Committee had invested millions in a vaunted turnout effort.

Did she lose because of weak turnout? The answer highlights the seemingly contradictory lessons of the 2014 midterm elections. If the electorate had been as young and diverse as it was in the presidential election year of 2012, Ms. Hagan would have won — but she still drew a stronger turnout than Democrats received in the 2010 midterms. Ms. Hagan probably didn’t perform well enough to win under anything but the most favorable turnout situation possible for a midterm electorate.