The Nikkei share average ended 0.9 percent lower to 21,480.90, its lowest close since March 9 and dropping below its 25-day moving average of 21,674.28.
The broader Topix shed 1.0 percent to 1,719.97, with 32 of its 33 subsectors in negative territory.
One opinion poll published on Sunday showed Abe's support falling to its lowest since he took office in 2012, while all polls showed that most Japanese believe Abe bears some responsibility for altered documents at the centre of suspicions that his government had tried to cover up possible cronyism in a land deal.
Exporters underperformed, with TDK Corp falling 1.5 percent, Taiyo Yuden shedding 2.4 percent and Panasonic declining 2.1 percent.
Brokerages were also sold, with Nomura Holdings falling 1.0 percent and Daiwa Securities Group sliding 1.5 percent.