Because Emperor Shijo had died in 1242, a prince of Emperor Juntoku, Chuseio, was to be enthroned as the new Emperor; however, Yasutoki strongly objected to this because the father, the Emperor Juntoku, had been a leader in the Jokyu War, and Yasutoki took an attitude that he would compel Chuseio to abdicate if his enthronement was realized; therefore, Yasutoki forcibly enthroned the new Emperor Gosaga against the complaints and opposition of the court nobles.
仁治3年（1242年）に四条天皇が崩御したため、順徳天皇の皇子・忠成王が新たな天皇として擁立されようとしていたが、泰時は父の順徳天皇がかつて承久の乱を主導した首謀者の一人であることからこれに強く反対し、忠成王即位が実現するならば退位を強行させるという態度をとり、貴族達の不満と反対を押し切って後嵯峨天皇を新たな天皇として即位させた。