Updated

TOKYO (AP) — A small party decided to leave Japanese Prime Minister Yukio Hatoyama's ruling coalition over his broken campaign promise to move a U.S. Marine base off Okinawa island, as he faced calls Sunday to resign and dim prospects in upcoming elections.

The departure of the Social Democratic Party from the three-party coalition is unlikely to bring down the government led by Hatoyama's Democratic Party of Japan. But his poor handling could significantly hurt the Democrats' performance in upper house elections expected in mid July.

"We have decided to leave the coalition government," Social Democratic party leader Mizuho Fukushima told reporters after meeting with party executives. "It was a tough decision to make, but a political party cannot do without public trust."

Hatoyama dismissed Fukushima on Friday as the minister for gender equality and consumer affairs after she refused to sign a Japan-U.S. agreement to move the American base to a less crowded part of Okinawa.

Hatoyama's decision to keep the base on Okinawa, broadly in line with a 2006 deal forged by the previous Japanese government, helped heal Tokyo's ties with Washington but broke the prime minister's campaign promise to move the base off the southern island. It has infuriated Okinawa residents who have long complained about the heavy U.S. military presence.

Fukushima, who has supported the Okinawans, said Friday that her dismissal meant Hatoyama had turned his back on Okinawa. "My dismissal is a betrayal of the people," she said.

Analysts say many voters sympathize with her, and Hatoyama would eventually have to step down — though the timing is still unpredictable — to minimize the damage ahead of elections, while Fukushima's small party could make a big gain.

"Many voters now see Hatoyama as a liar who fired an honest minister who stood by her principles," said Takehiko Yamamoto, international politics professor at Waseda University. "Prime Minister Hatoyama has lost credibility over what he says. He would have to step down to contain damage. If he lingers on, it would mean less support for the Democrats."

Party spokesman Manabu Ito said Fukushima was expected to officially hand Hatoyama the party's decision to divorce the ruling bloc.

Cabinet ministers had mixed reaction.

Reform Minister Yoshito Sengoku said it was "unfortunate" to lose the coalition partner. Transport Minister Seiji Maehara welcomed the outcome, saying "Each of us are now following own policies and that's the way it should be."

Hatoyama had urged Fukushima's party to stay in the coalition. But Social Democrat deputy chief Seiji Mataichi told reporters earlier Sunday that "Hatoyama has lost rational judgment and is not capable of living through a crucial situation. He no longer qualifies as leader of the country."

Calls for his resignation are growing elsewhere too.

"Because he broke his promise, he has no choice but resign," said opposition Liberal Democratic Party secretary general Tadamori Oshima.

Kyodo News agency said its weekend telephone survey of 1,033 voter households showed more than half saying Hatoyama should step down, while 44 percent disagreed. Kyodo did not give a margin of error, but a poll of that size would normally have a 5 percentage point sampling error.

Hatoyama's Democrats have 310 seats in the 480-member lower house, while the Social Democrats have seven. The Social Democrats also hold five seats in the less-powerful upper house, where Hatoyama's party and another coalition partner together have just over a majority.

Half of the 242-seat upper house would be up for re-election. Analysts predict the Democrats and its another coalition partner could lose from 15 to 20 seats and would be short of a majority in the house needed for smooth passage of legislation.

Independent political analyst Takao Toshikawa said Hatoyama could also face calls for resignation from his own party members.

"Democratic candidates fear they cannot win elections under Hatoyama," he said.

Hatoyama repeatedly apologized Friday for failing to keep his pledge to move the base off the island, which hosts more than half the 47,000 U.S. troops stationed in Japan under a 1960 joint security pact.

He said Futenma's helicopter and air assets were needed for nearby U.S. Marine infantry units on Okinawa in times of emergency, citing the recent sinking of a South Korean warship blamed on North Korea.