On Thursday, the new party picked CDPJ leader Yukio Edano as its leader and voted to retain the CDPJ label for the new party.
Edano urged the government to open an extra session of parliament for debate but added: "If for their own selfish reasons they run away from this, we will face them head-on and give the people a choice."
The new CDPJ will have 149 incumbent members of parliament, compared with the LDP's nearly 400.
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Support for the LDP dwarfs that for the new opposition group, although the merger has improved its backing.
A Kyodo news agency survey released on Wednesday showed 48.1 percent would vote for the LDP against 15.7 percent for the merged party, up from just 4 percent who backed the CDPJ in a recent poll.
(Writers: Ritsuko Ando, Linda Sieg | Editor: Gerry Doyle)
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