SYDNEY — Prime Minister Howard suffered a humiliating defeat Saturday at the hands of the left-leaning opposition, whose leader has promised to immediately sign the Kyoto Protocol on global warming and withdraw Australia’s combat troops from Iraq.

Labor Party head Kevin Rudd’s pledges on global warming and Iraq move Australia sharply away from policies that had made Mr. Howard one of President Bush’s staunchest allies.

Mr. Rudd has named global warming as his top priority, and his signing of the Kyoto Protocol will leave America as the only industrialized country not to have joined it.

Mr. Rudd said he would withdraw Australia’s 550 combat troops from Iraq, leaving twice that number in mostly security roles. Mr. Howard had said all the troops will stay as long as needed.

Official figures from the Australian Electoral Commission showed Labor far in front after more than 70% of the ballots had been counted — with 53% of the vote compared to 46.7% for Mr. Howard’s coalition.

Using those figures, an Australian Broadcasting Corp. analysis showed that Labor would get at least 81 places in the 150-seat lower house of Parliament — a clear majority.

It was an embarrassing end to the career of Mr. Howard, Australia’s second-longest serving leader.

As little as a year ago, Mr. Howard had appeared almost unassailable. But on Saturday he was in real danger of becoming only the second sitting prime minister in 106 years of federal government to lose his own seat in Parliament.

Mr. Howard took full blame for the drubbing handed to his center-right coalition.

“I accept full responsibility for the Liberal Party campaign, and I therefore accept full responsibility for the coalition’s defeat in this election campaign,” Mr. Howard said in his concession speech in Sydney.

A new government is unlikely to mean a fundamental change in Australia’s close alliance with America — its most important security partner — or its growing economic and political ties with Asia.

At home, Mr. Rudd has pledged to govern as an “economic conservative,” while pouring money into schools and universities. He will curtail sweeping industrial reforms laws that were perceived to hand bosses too much power, turning many working voters against Mr. Howard.

“Today Australia has looked to the future,” Mr. Rudd said in a nationally televised victory speech, to wild cheers from supporters. “Today the Australian people have decided that we as a nation will move forward … to embrace the future, together to write a new page in our nation’s history.”

In his concession speech, Mr. Howard announced he had phoned Mr. Rudd to congratulate him on “a very emphatic victory.”

The change from Mr. Howard to Mr. Rudd also marks a generational shift for Australia.

Mr. Rudd, a 50-year-old former diplomat who speaks fluent Chinese, urged voters to support him because Mr. Howard, 68, was out of touch with modern Australia and ill-equipped to deal with new-age issues such as climate change.

Mr. Howard campaigned on his economic management, arguing that his government was mostly responsible for 17 years of unbroken growth, fueled by China’s and India’s hunger for Australia’s coal and other minerals, and that Mr. Rudd could not be trusted to maintain prosperous times.

Labor has been out of power for more than a decade, and few in Mr. Rudd’s team — including him — has any government experience at the federal level. His team includes a former rock star — Midnight Oil singer Peter Garrett — a television journalist and former union officials.

But analysts say his foreign policy credentials are impeccable, and that he has shown discipline and political skill since his election as Labor leader 11 months ago.

Mr. Rudd’s election as Labor leader marked the start of Mr. Howard’s decline in opinion polls, from which he never recovered.

Mr. Howard’s four straight election victories since 1996 made him one of Australia’s most successful politicians. He refused to stand down before this election — even after being urged to do by some party colleagues.