BRUSSELS — What if the greatest threat to the European Union is not Russian aggression, Chinese exports, or US President Donald Trump, but actually comes from within the bloc? Such a scenario becomes more likely by the day, as illiberal, nationalist forces grow stronger ahead of elections in Spain, France, Poland, and several German states. If these parties come to power, they could hollow out the EU from within, undermining its fundamental values and obstructing collective action. Supporters of the bloc in its current liberal-democratic form must recognize the gathering storm and be prepared with new political strategies and institutional arrangements. For decades, European integration has rested on the assumption that, despite recurring crises, member states were committed to the EU’s survival and success. That assumption no longer holds. Anti-European forces now challenge not just EU policies, but the underlying logic of integration, from shared sovereignty to common rules. But instead of seeking to dissolve or exit the bloc, Europe’s far right and populists have moved on to an