Speculation that Tokyo Gov. Yuriko Koike may run for a Lower House seat is fast losing steam, as the governor dismissed the prospect of her return to the Diet in the Oct. 22 election as "100 percent impossible."

Ever since her surprise establishment of the new political party Kibo no To (Party of Hope) last week, Nagatacho, the political epicenter of the nation, has been awash with rumors that Koike may step down as governor and instead wage an all-out war against Prime Minister Shinzo Abe by running for a seat herself.

In Japan, only a Diet member is eligible to become prime minister, according to the Constitution.