TOKYO -- Support for Japan's government fell further ahead of a looming election, polls showed on Monday, as calls emerge within his party for struggling Prime Minister Yukio Hatoyama to quit over broken campaign promises.

Hatoyama reiterated he would do his best in his current post, but chances are fading for his ruling coalition to win a majority in an upper house election expected in July, which it needs to win to avoid policy deadlock.

The political furore is distracting the government as it thrashes out plans to cut Japan's huge public debt and a strategy to engineer growth despite a fast-ageing population.

Japanese newspaper polls showed fresh declines in voter support for Hatoyama's eight-month-old government, with an Asahi newspaper survey showing its rating slid to 17 percent, down from 21 percent a few weeks ago.

In the latest blow to the struggling Hatoyama, the tiny Social Democratic Party (SDP) left the ruling coalition on Sunday after the premier abandoned his pledge to move a US Marine airbase off Japan's southern Okinawa island.

While the departure of the SDP could deprive the Democrats of some votes in the upper house election, it will not topple the government since the Democrats have a huge majority in parliament's more powerful lower house.

"It is very disappointing that we could not agree on fundamental thinking on national security," Hatoyama said of the SDP's departure. "But we need to overcome this by being determined and by me doing my best to bring back reliable politics for the Japanese people," he told reporters.

Most opinion polls showed a majority of voters think Hatoyama should step down for failing to keep his campaign promise on the airbase, and even some in his own party want him to resign.

Democratic Party elder Kozo Watanabe said on Saturday he hoped Hatoyama would "make up his mind for the sake of the future of the country and its people," a thinly disguised call to quit.

But time is running out to replace Hatoyama ahead of the upper house poll, expected on July 11. And some in his party also worry changing the prime minister so soon after sweeping to the power would remind voters of previous LDP-led governments, two of whose leaders threw in the towel after about a year in office.

Hatoyama's government has wobbled on a range of promises, from cash allowances for parents of young children to abolishing highway tolls.