In Poland, next month’s parliamentary elections may be the opposition’s last, best chance to stop the country’s slide into autocracy. Along with Hungary, Poland once counted as a paradigmatic success story for a postcommunist transition to democracy. But also like Hungary, that reputation started to sour when far-right populists surged to power in the 2010s. What happens in Poland is the more consequential because it is by far the largest Central or Eastern European country in the European Union. Its location—bordering Ukraine, Belarus, the Russian enclave of Kaliningrad, and the Baltic Sea—gives it immense geopolitical importance. It has a more powerful military than neighboring Germany. And according to some projections, its GDP per capita is even set to overtake Britain’s by the end of the decade. The populist Law and Justice party secured a majority in Poland’s parliament, and won the largely ceremonial presidency, in 2015. Soon after, Jarosław Kaczyński, the party’s leader, who is widely understood to exercise the real power in the land, held a long meeting with Hungary’s prime minister, Viktor Orbán—and …