If the ruling Liberal Democratic Party-New Komeito coalition was the big winner in Sunday's elections, the little winner was the coalition's most dogged critic, the 91-year-old Japanese Communist Party.

Early exit polls showed the JCP winning around eight seats of the 121 up for election in the upper house. If that number materializes, it would be their biggest upper house win since 1998, when the party took 15 seats.

Has...