Top executives from two opposition parties reached a broad agreement Monday to form a joint parliamentary group, a move that could potentially knock the Constitutional Democratic Party of Japan — a staunchly liberal party dead-set against Prime Minister Shinzo Abe’s bid to revise the Constitution — out of its position as the biggest opposition force in the Lower House.

The deal by Kibo no To (Party of Hope) and the Democratic Party, if finalized, would mark a tectonic shift in the Diet since the No. 1 opposition party is usually given a greater say in parliamentary decisions, including how to proceed on constitutional amendments should any be proposed.

On Monday, the secretaries-general of the two parties met and agreed to tie up for this year's regular Diet session, which kicks off on Jan. 22.