Japan's 4-year-old governing coalition fractured Saturday, as the Social Democratic Party formally announced that it would quit and strike out on its own.

It is a measure of how far the Social Democrats have fallen in popularity that their departure will scarcely matter. The Liberal Democratic Party, which has dominated Japanese politics since the 1950s, will continue to govern Japan, with no need for new elections or a Cabinet reshuffle.The collapse of the coalition had been expected and was not expected to cause any particular instability. The Liberal Democrats have a slight majority in the House of Representatives, the more important chamber of the Japanese Parliament, and therefore believe they do not need the coalition to maintain power.

In addition, the Social Democratic leader, Takako Doi, said that her party would support the government of Prime Minister Ryutaro Hashimoto in a no-confidence motion that the opposition parties are planning. Doi said that her party could not vote to condemn government policies that it had helped to formulate. "We can hardly say we are not responsible" for current problems, she said.

The coalition, formed in June 1994, had always been an improbable blend, a triumph of ambition over principle. It united the Liberal Democrats, who are more like what in the United States would be called conservative Republicans, with the Social Democrats, who were then known as the Socialist Party.

Ironically, nothing so undermined the Social Democrats as proximity to power. For decades they had formed the main opposition party in Japan, and there was occasional talk that they might even be elected to form a government on their own.

In contrast, they are now almost irrelevant, and it is unclear how much longer the party will even survive.