Dhaka, July 20 -- Japanese voters headed to the polls Sunday to elect members to the country's upper house of parliament, in a high-stakes election that could deal a blow to Prime Minister Shigeru Ishiba and his ruling coalition, heightening political uncertainty.

The vote will determine half of the 248 seats in the upper house - the less powerful of the two chambers in Japan's legislature. Preliminary results are expected later Sunday.

Prime Minister Ishiba has set a modest goal: securing a simple majority of 125 seats. His Liberal Democratic Party (LDP) and junior coalition partner Komeito, which already hold 75 seats, need to win 50 more. This target marks a sharp drop from the 141 seats the coalition held before the election, and op...