Former Liberal Democratic Party kingmaker Mikio Aoki died Sunday. He was 89.

Aoki, known as the “don of the Upper House” for his influence inside and outside the LDP, served as a lawmaker in the chamber between 1986 and 2010. He began his political career as a secretary to Noboru Takeshita, who later became prime minister, and went on to serve as a prefectural assembly member in Shimane Prefecture, where Takeshita hailed from, before entering national politics.

“Aoki earned the trust of those beyond the ruling and opposition parties. I respect his stance of valuing that trust in his work as a politician,” Chief Cabinet Secretary Hirokazu Matsuno told a news conference Monday.

With his strong connections with Komeito, Aoki played a leading role in creating the LDP's coalition government with the party, which began in 1999.

Aoki was chief Cabinet secretary when Prime Minister Keizo Obuchi had a stroke and fell into a coma in April 2000, creating a leadership crisis. Obuchi died the following month.

At the time, Japan lacked a clear line of succession in the event that a prime minister died or was unable to perform their duties.

On April 2, 2000, after Aoki visited Obuchi in the hospital right after the latter’s collapse, he claimed that Obuchi briefly came out of the coma and appointed him as acting prime minister. Obuchi’s condition worsened and he never regained consciousness, eventually dying on May 14 at the age of 62.

Doubts over Aoki’s claim to have been appointed acting prime minister remained for years.

Aoki was also among the five key LDP members to essentially handpick Obuchi’s successor — then-LDP Secretary-General Yoshiro Mori — triggering a controversy about the new prime minister’s legitimacy.

That controversy did not impact Aoki’s power within the Upper House or the LDP, however. Even after retiring from politics, he continued to have a say in his LDP faction, which is now headed by Secretary-General Toshimitsu Motegi. Aoki served as an informal advisor to both the 54-member faction and successive prime ministers.

While Motegi wants to remain head of the faction and is believed to have his eyes on the position of LDP president, which would likely set him up to become prime minister, Aoki had been pushing Yuko Obuchi, the former prime minister’s daughter who also belongs to the Motegi faction, creating friction with Motegi.

Aoki also became known in the political world for devising the “Aoki formula” for predicting the fate of prime ministers. The general rule of thumb for the formula is that if the sum of the Cabinet approval rating and the LDP’s support rate falls below 50%, the prime minister cannot maintain power and will be forced to step down.

The formula continues to influence how the LDP and the prime minister judge the mood of the country and whether to have Cabinet and leadership reshuffles, or a general election.