Europe’s antiestablishment politicians have barely been able to contain their excitement at Donald Trump’s election victory. From France’s Marine Le Pen to Hungary’s Viktor Orban to the U.K.’s Nigel Farage, they have greeted the U.S. president-elect’s triumph as a validation of their own longstanding opposition to immigration and free trade. With a series of political tests looming in Europe over the next year—starting with next month’s Austrian presidential election and Italian constitutional referendum and followed by elections in the Netherlands, France and Germany in 2017—European populist parties now sense an opportunity...