Ambrose emerged as a leading advocate of papal primacy. This attitude he showed clearly when he attended the Council of Basel as legate of Pope Eugene IV and defended the primacy of the pope, adjuring the council not to "rend asunder Christ's seamless robe". He was next sent by the pope to the Emperor Sigismund to ask his aid in the pope's efforts to end this council, which for five years had been encroaching on papal prerogatives. The pope transferred the council from Basel to Ferrara on 18 September 1437.