Fourteen million Australians face the possibility this weekend of being led by their second left-wing Labor government in 31 years.

Trapped into running a relatively lackadaisical campaign by overconfidence, the Conservative coalition government of Prime Minister Malcolm Fraser is behind Labor in every significant opinion poll by as much as 7 percent.

He is spending the last week before the election mounting an all-out attack on the opposition. He accuses the Labor Party of being controlled by its socialist left wing and predicts that a Labor government would spend so much on welfare that inflation would shoot to 20 percent.

Although opinion polls for the past 12 months had consistently put Labor ahead, observers on both sides were confident that Fraser, two landslide election wins behind him since he gained power in 1975, would coast to victory.

He had most factors in his favor. He had the choice of election date and went for Oct. 18, two months early and before potentially damaging inflation figures were issued and before school graduates would boost the relatively high, 6 percent unemployment rate.

Fraser could also boast -- and has throughout the campaign -- that under his leadership Australia has gotten its inflation rate down to 10 percent, one of the lowest in the industrialized world.

Fraser went into the campaign with one extra advantage: He and most of the country thought that the self-effacing and little-known Labor Party leader, William George Hayden, would be a hopeless campaigner at his first try for national leadership.

But Hayden proved to be the campaign sleeper. He shucked off the nervy, high-pitched voice that had made him the "whining Willie" of the national parliament for three years and came out with a crisp, hard-hitting style.

Treasurer in the final year of the 1972-75 Gough Whitlam Labor government and elected Labor leader when Whitlam quit politics after his 1977 election thrashing, Hayden translated his reputation for economic moderation into a program designed to boost the Australian economy, but not too much.

He promised an across-the-board tax cut of $3.40 a week and government-service job creation programs designed to absorb a quarter of the nation's 400,000 jobless within a year. All children under 16 and pregnant women would get free medical care, and a Labor government would create a national plan for free medical care within three years. Hayden promised $120 million a year to upgrade schools in underprivileged areas.

Fraser promised a continuation of tight budgets and curbs on the money supply.

Bred from Australia's rural aristocracy and a millionaire sheep and cattle rancher, Fraser is as aloof from most easygoing Australians as his height, 6 feet 6 inches, physically places him above them. He has a reputation he does not dispute for being a bully with his Cabinet ministers and staff. Few Australians have ever liked him, and during this campaign they seem to be transferring their dislike into votes for his opponent.

The outcome is likely to be close. Hayden needs to win big to overcome the 49-seat majority Fraser holds in the national Parliament's House of Representatives.

Fraser may be lucky. He has pampered rural voters with subsidies for farmers and such election pleasers as a communications satellite to bring color TV to every home, even in Australia's Outback.

But Bill Hayden, who was once an Outback policeman, has a real chance of becoming Australia's prime minister Saturday.