The Democratic Party of Japan began testing its mettle Thursday as the top force in the Upper House by submitting during the Diet's extra four-day session two bills aimed at challenging ruling bloc-penned laws.

The first bill limits use of pension insurance premiums for just pension payments. The law the Liberal Democratic Party-New Komeito ruling bloc rammed past the DPJ in the last ordinary Diet session permits the money to be used for other purposes, including pension-related publicity.

Later in the day, the DPJ teamed up with the Social Democratic Party and Kokumin Shinto (People's New Party) to submit a bill to postpone the privatization of the postal service. Although not spelled out in the bill, the parties are eyeing a one-year delay to revise the privatization plan, which is to start Oct. 1.