Voters increasingly want Democrats to control Congress after the 2018 elections, according to a new Wall Street Journal/NBC News poll that offers several warning signs for the Republican Party.

Asked which party they prefer to lead Congress after next year’s midterms, 50% said the Democrats and 39% said Republicans. That 11-point lead is wider than the 7-point advantage Democrats held in October, and it is the first double-digit advantage for the party since late 2008, ahead of the Democrats’ win in the presidential election...