A former leader of Germany’s Social Democrats (SPD) has suggested the party could quit Chancellor Angela Merkel’s ruling coalition over the nomination of Defense Minister Ursula von der Leyen as European Commission chief.

Sigmar Gabriel told Der Spiegel that “if Merkel names von der Leyen without a cabinet decision, that is a clear violation of the rules of the federal government - and a reason to leave the government.”

Merkel said on Tuesday she abstained in a vote of EU leaders on von der Leyen’s nomination – the only leader not to back her – as the coalition government was at odds on the question, Reuters reports.

Gabriel says von der Leyen’s nomination is an “unprecedented act of political trickery.”