TOKYO (AP) — Outspoken leaders from Japan’s two biggest cities formed a national political party Saturday, seeking to become "a third force" to lure undecided voters and challenge the country’s two biggest parties.

Nationalist Shintaro Ishihara, who resigned as Tokyo governor to create his own party this week, said he is scrapping his four-day-old group to join the Japan Restoration Party formed in September by the young and brash mayor of Osaka, Toru Hashimoto.

The announcement comes the day after Prime Minister Yoshihiko Noda dissolved the lower house of parliament, paving the way for Dec. 16 elections. His ruling party is expected to give way to a weak coalition government divided over how to tackle Japan’s myriad problems. The biggest problems are getting a stagnant economy going again and reconstruction after the crippling March 2011 earthquake and tsunami.

Elections are set for Dec. 16, with official campaigning starting Dec. 4. If Noda’s centrist party loses, the economically sputtering country will get its seventh prime minister in six and a half years.

Japan is going through a political transition with a merry-go-round of prime ministers and the mushrooming fringe parties to challenge the long-dominant Liberal Democrats’ return.

"This country is going to fall apart if we don’t act now," Ishihara told Saturday’s party convention held in Osaka, announcing the merger of his party and Hashimoto’s. "I’ve decided to ignore small differences to join hands on common ground. This will be my last service for the country."

Apparently Ishihara made concessions to Hashimoto’s policy supporting phase-out of nuclear energy and participating in the U.S.-led trans-Pacific trade block. The timing of the election could pre-empt moves by more conservative challengers to build enough electoral support.

Ishihara, 80, now heads the Japan Restoration Party, replacing Hashimoto, who now serves No. 2 post. Hashimoto, 43, has said he will remain mayor of Japan’s second-largest city and not run in the election. On Saturday, he announced backing 46 candidates to run in the polls.

"We’ll claw our way through the election battle — not just to win seats, but to change the root of this country," Hashimoto told a televised party convention. "We will change all forces that try to defend the status quo."

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

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 due to stomach ailment.

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.

Ishihara said his party aims to be "a second force" not third, to be close enough to take power.

Ishihara resigned as Tokyo governor and created the Sunrise Party with several ultra-conservative national lawmakers Tuesday. As governor, Ishihara helped instigate the territorial spat with China by saying Tokyo would buy and develop the disputed East China Sea islands controlled by Japan but claimed by Beijing. The central government bought the islands, apparently to thwart Ishihara’s more inflammatory plans, but failing to calm China’s anger.