Tokyo Gov. Yuriko Koike officially became the leader of a political group supporting her Thursday and bolted the conservative Liberal Democratic Party the same day.

The 64-year-old governor took the helm of Tomin First no Kai (Tokyoites First) at the group's campaign launch for the Tokyo Metropolitan Assembly election set for July 2.

Tomin First no Kai, widely seen as a potential rival to the LDP in the assembly, had endorsed 48 candidates as of late May as it looks to secure a majority in the 127-seat assembly through an alliance with Komeito and other political forces close to Koike.

"Although this is still a local party, I put myself at the forefront of it in a resolute manner," she told a news conference.

A veteran politician who has served as environment minister and, briefly, as defense minister as a member of the LDP, Koike became Tokyo's first female governor last August. She defeated contenders that included an LDP-backed candidate but at that time stopped short of formally leaving the party, which is headed by Prime Minister Shinzo Abe.

Koike said she notified LDP Secretary-General Toshihiro Nikai, the party's No. 2 man, of her intentions in advance. Nikai simply told her, "Have a good fight," she said.

A weekend poll by Kyodo News showed 11 percent of the respondents said they may vote for Tomin First no Kai and 17 percent said they will vote for the LDP, with the bulk of voters apparently undecided.

Chief Cabinet Secretary Yoshihide Suga played down Koike's move Thursday.

"I don't think it could have an impact" on the campaign for the Tokyo assembly election.

The political group was formed last September by Tokyo assembly members supporting Koike and was headed by Kazusa Noda, a special secretary to the governor who was serving as special adviser.