TOKYO - Japan's tiny Social Democratic Party (SDP) decided on Sunday to leave the ruling coalition ahead of an election, as unpopular Prime Minister Yukio Hatoyama faced calls to step down over broken campaign promises.

The departure of the SDP is a blow to Hatoyama, already seen by voters as a weak leader, damaging his Democratic Party's chances of winning a majority in an upper house election expected in July, which it needs to pass bills smoothly.

It does not force the Democrats out of power since they boast a massive majority in parliament's more powerful lower house, but may distract from the government's struggle to rein in debt while nurturing a fragile economic recovery.