For weeks, the nation's two largest opposition parties have been working on a merger, aiming to establish a formidable alternative to the ruling bloc led by Prime Minister Shinzo Abe ahead of the start of the ordinary Diet session set to begin Monday.

Sources who spoke to The Japan Times, as well as reports citing lawmakers by multiple media outlets, suggest the merger is unlikely to be completed ahead of the session, squandering what many observers see as a golden opportunity for the Constitutional Democratic Party of Japan and the Democratic Party for the People to form a united front in the event of a snap election.

Opposition parties have been realigning themselves for years, but so far have been unable to come together — a weakness Abe has used to his advantage. Since he became prime minister in late 2012, his Liberal Democratic Party and coalition partner Komeito have trounced the opposition in poll after poll.