The Social Democratic Party has told the Liberal Democratic Party that it will withdraw from the ruling alliance during the current Diet session -- a move that could affect the balance of power in the Upper House -- if the LDP does not compromise and support a proposed political ethics bill.

SDP Secretary General Shigeru Ito on Tuesday informed Yoshiro Mori, LDP executive council chairman, and Hiromu Nonaka, the party's acting secretary general, of the decision, according to sources.

But the LDP has no intention of compromising on the anticorruption bill -- which would prohibit lawmakers from obtaining money through influence-peddling -- a stance that seemingly makes the SDP's departure inevitable.

The SDP has maintained its alliance with the LDP for four years. The LDP holds a majority in the Lower House but needs to maintain its alliance with the SDP and New Party Sakigake to also dominate the Upper House.

Nonaka, according to the sources, hinted that an SDP departure could trigger elections for both chambers of the Diet -- a development the SDP does not want. The three agreed that Ito and LDP Secretary General Koichi Kato should discuss the matter today when Kato returns from the U.S.

In Washington, Kato said that if the SDP follows through with its threat, he will urge the SDP to join a "policy-based alliance" after the Upper House elections in July. In a meeting with Japanese reporters, Kato said the SDP would top the list of candidates that the LDP would ask to join a policy-based, postelection alliance.

Other candidates include Sakigake, the Heiwa Kaikaku group (New Peace Party and Reformers' Network Party) and Komei, Kato said.