Seven lawmakers said Tuesday they would leave their parties to join a new grouping being formed by populist Osaka Mayor Toru Hashimoto, a man seen as possible kingmaker in Japan's next general election.

The outspoken and colorful Hashimoto is increasingly a force to be reckoned with on Japan's often grey political stage.

His abrasive wit and outsider status set him apart from the air of entitlement among establishment lawmakers, many of whom are second or third generation parliamentarians.

An almost nightly fixture on news programs, he is the go-to man for broadcasters looking for a lively quote on everything from Korean sex slaves to tattooed public servants.

Three people resigned from the ruling Democratic Party of Japan (DPJ) on Tuesday, with four others taking their leave of opposition parties.

"I submitted my resignation as I want to do the things that I couldn't before," Yorihisa Matsuno, a DPJ member who once served as deputy chief cabinet secretary, told reporters, adding that he had "felt the limitations of the established parties."

Hashimoto, 43, arguably Japan's most keenly-watched politician, is expected to formally announce the establishment of Nippon Ishin no Kai (Japan Restoration Association) as early as Wednesday.

"I'm heading into a new stage with the determination that we'll change the country," Hashimoto said on Sunday as he announced his effective entry into national politics.

He has said he plans to double as mayor of Osaka, the hub city in western Japan, and the head of the new national party.

His new party is considered a serious threat to the DPJ and the main opposition Liberal Democratic Party (LDP) in looming general elections.

Public support ratings for Prime Minister Yoshihiko Noda's government are hovering below 30 percent, battered by politically costly legislation to double sales tax and beset by territorial spats with China and South Korea.

But the establishment LDP, which ruled Japan for more than 50 almost unbroken years, has been unable to capitalize on Noda's unpopularity.

A vague promise it extracted from the prime minister that he would hold a general election "in the near future" in exchange for support of his tax bill left it looking opportunistic.

Observers say neither party will win a majority in polls, likely to take place in the autumn, and Hashimoto is expected to be courted in the coalition-building that looks set to follow a popular ballot.