He was a prominent faction leader in the Liberal Democratic Party (LDP) during the late 1990s and early 2000s, and served as its Secretary-General and Vice President under Prime Minister Junichiro Koizumi.

As head of the LDP Policy Research Council, he advocated investing government-controlled postal savings and insurance funds into the stock market, and an escalation in government spending on telecommunications, environmental and education projects.

[8] Yamasaki was a vocal supporter of Koizumi's reform efforts, which targeted the LDP's traditional pork barrel constituencies.

His district in urban Fukuoka Prefecture, with an electorate that frequently moved in and out of the region for work, was a favorable battleground for opposition candidates.

[10] In September 2002, Yamasaki's fundraising office was reported to have passed donations from construction companies to Yasushi Kaneko, an independent lawmaker supporting the Kawabe Dam project in Kumamoto Prefecture.

[12] In the November 2003 election, Yamasaki was defeated by Jun'ichirō Koga of the Democratic Party, and subsequently resigned from the vice-presidency of the LDP.

In the run-up to the 2009 general election, Yamasaki and Kato considered forming a new party to challenge the beleaguered LDP, and had discussions with both Shizuka Kamei and Ichiro Ozawa.