The leading candidate, Friedrich Merz, a conservative who has adopted many of the AfD’s hard-line positions on immigration, used his closing statement of the debate to promise his voters that he would never allow the AfD into his government. Under him, he said, the firewall would hold.

Germany’s federal elections took place this weekend, with the conservative Christian Democrats (CDU) and their Bavarian sister party the Christian Social Union (CSU) winning overall and set to form a coalition government.

The leader of the far-right Alternative for Germany (AfD), the second largest party in parliament after Sunday's election, said Germany's likely next chancellor would be held hostage by left-wing parties that would seek to loosen tough fiscal rules.