He opposed the privatization of the postal service pushed by Prime Minister Junichiro Koizumi.

[3][4] Like many other postal rebels, Moriyama returned the LDP under Koizumi's successor Shinzo Abe in December 2006.

In July 2007 he joined the Kinmirai Seiji Kenkyūkai faction led by Taku Yamasaki.

Moriyama served as the Minister of Agriculture, Forestry and Fisheries in the cabinet of Shinzo Abe from October 2015 to August 2016.

He was noted as a close ally of Suga and Secretary General Toshihiro Nikai.

In August 2022 he was appointed chairman of the Election Strategy Committee in the LDP, putting him in charge of the difficult task of coordinating candidates after a recent redistricting.