BERLIN — Germany and Turkey have been locked in an intensifying war of words over the past week, as campaigning heats up before an April referendum in Turkey on a new Constitution that would expand the powers of President Recep Tayyip Erdogan.

Mr. Erdogan, whose critics cast him as ever more authoritarian, badly wants a victory in the vote. With the referendum on a knife-edge, he and members of his Justice and Development Party, known as A.K.P., are desperate to campaign in Germany among the 1.5 million Turks who are eligible to vote.

“There is a need for the A.K.P. to secure as many votes as possible from the Turks living in Germany — that’s the basic ingredient,” said Marc Pierini, a former European Union ambassador to Turkey and a scholar at Carnegie Europe, a Brussels-based think tank.

“The yes vote is now in jeopardy, therefore votes in Germany are of course very important,” he added.