The administration under Japanese Prime Minister Shinzo Abe will face a major political test when the House of Councilors election opens on Sunday, as the ruling and opposition camps are rivaling for majority in the upper house.

On Sunday, 121 of the 242 six-year seats in the upper chamber are up for grabs: 73 in prefectural constituencies and 48 proportional representation seats.

Several recent media surveys showed that the ruling coalition is fighting an uphill battle to maintain its majority in the upper house. The Liberal Democratic Party (LDP), headed by Abe and its coalition partner the New Komeito party needs to secure 64 seats to keep its majority.

Meanwhile, the largest opposition Democratic Party of Japan (DPJ) seemed to have gathered considerable momentum to change the landscape. According to a survey conducted by the Asahi Shimbun newspaper on Tuesday and Wednesday, the LDP will likely fall short of 44 seats. Meanwhile, the DPJ could win as many as 60.

However, LDP is ahead in overall support rates of the political parties in several surveys. The unaffiliated voters, who may not decide whether to vote or how to cast the ballots, are thought as an important factor towards the final result.

Before the election on Sunday from 7:00 a.m. (2200 GMT, Saturday) to 8:00 p.m. (1100 GMT), some 4 million absentee ballot shave been received as of last Sunday. The number of eligible voters in Japan stands at around 104 million.