Prime Minister Yoshihiko Noda dissolved the Lower House of Parliament on Friday, paving the way for elections in which his ruling party will likely give way to a weak coalition government divided over how to solve Japan’s myriad problems.

Elections were set for Decr 16. If Mr. Noda’s centre-left party loses, the economically sputtering country will get its seventh Prime Minister in as many years.

The opposition Liberal Democratic Party, which led Japan for most of the post—World War II era, is in the best position to take over. The timing of the election likely pre-empts moves by more conservative challengers, including former Tokyo Mayor Shintaro Ishihara, to build up electoral support.

Campaigning is set to begin Dec. 4, but leaders were already switching into campaign mode.

“What’s at stake in the upcoming elections is whether Japan’s future is going to move forward or backward,” Mr. Noda declared to fellow leaders of the Democratic Party of Japan. “It is going to be a crucial election to determine the fate of Japan.”

The DPJ, in power for three years, has grown unpopular thanks largely to its handling of the Fukushima nuclear crisis and its recent doubling of the sales tax.

Mr. Noda’s most likely successor is LDP head and former Prime Minister Shinzo Abe. He resigned as Japan’s leader in 2007 after a year in office, citing health problems he says are no longer an issue.

The path to elections was laid suddenly on Wednesday during a debate between Mr. Abe and Mr. Noda. Mr. Noda abruptly said he would dissolve Parliament if the opposition would agree to key reforms, including a deficit financing bill and electoral reforms, and Mr. Abe jumped at the chance. Polls indicate that the conservative, business—friendly LDP will win the most seats in the 480—seat lower house but will fall far short of a majority. That would force it to cobble together a coalition of parties with differing policies and priorities.

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