In 1866, he tried to summon personnel of various domains in the name of the Imperial court when Iemochi TOKUGAWA died, but this attempt failed. When Emperor Komei died, there was a rumor that he was poisoned and Tomomi was suspected of killing him (there has been a theory that Tomomi assassinated Emperor Komei in order to raise his own position so that he could make young and easily manipulable Emperor Meiji take over the throne).
慶応2年（1866年）の徳川家茂の死を機会に朝廷の名において列藩召集を行なおうとするが失敗、孝明天皇の崩御の際には毒殺説が流れ、首謀者として疑われた（一説に自分が成り上がろうとして孝明天皇を暗殺し、幼く操縦しやすいと思われる明治天皇を早く即位させ利用した、という説もある）。