Prime Minister Fumio Kishida and the ruling coalition saw split results in key Upper House by-elections Sunday night, winning as expected in Yamaguchi Prefecture but losing a tough race to an opposition-backed candidate in Shizuoka Prefecture, according to local reports — a result that might benefit the opposition parties just one week before the Oct. 31 general election.

In Shizuoka, Shinnosuke Yamazaki, 40, a former independent prefectural assemblyman who was supported by the opposition Constitutional Democratic Party of Japan and the Democratic Party for the People, defeated ruling Liberal Democratic Party-backed former Gotemba mayor Yohei Wakabayashi, 49. A third candidate, Chika Suzuki, 50, was backed by the Japanese Communist Party.

In Yamaguchi Prefecture, the LDP’s Tsuneo Kitamura, 66, also backed by Komeito, defeated JCP candidate Kiyo Kawai, 61, and Ryu Hezuma, 30, an independent. The ruling coalition had held both Upper House seats prior to the by-election.

The victory by Yamazaki in the Shizuoka race comes as the CDP and the JCP are supporting unified candidates in a number of Lower House races for the first time ever. The DPP, citing differences with the JCP, refused to cooperate in the Lower House election but joined the CDP in backing Yamazaki, who won despite the fact that the presence of the JCP’s Suzuki meant a split opposition vote.

For the ruling coalition, the loss in Shizuoka could spell trouble in the Lower House election, as it was seen as the first test of Kishida’s administration since it was inaugurated earlier this month.