TORONTO, AUG. 27 -- Not since waves of Trudeaumania swept Canada in the late 1960s has there been a political phenomenon here like David Peterson, Ontario's premier.

Women swoon over him. Men nod approvingly at his purposeful, low-key patrician style. Polls predict that he will lead his Ontario Liberal Party to a landslide victory in elections Sept. 10. Pundits tout him as a strong contender for prime minister of Canada.

The saga of Peterson's sudden rise is one of Canada's favorite Cinderella stories. He put his Ontario Liberal Party in power, after 42 years out of office, by forging an unusual alliance to lead a minority government two years ago.

The year before, he had undergone a nearly total makeover.

He had resembled a "before" photograph in a dieting ad. Overweight, he wore baggy corduroy suits, long sideburns and black-rimmed eyeglasses with lenses as thick as the bottoms of old Coke bottles. He stammered when he spoke. And he faced defections in the ranks of his perennially losing party.

After Prime Minister Pierre Elliott Trudeau left office three years ago and the federal Liberal Party was routed in national elections, many of the canny political operatives who had advised Trudeau during his 16 years in power left Ottawa to go into private business in Toronto. They began advising Peterson, guiding him through a metamorphosis into "the yuppie premier," as he is fondly called.

A speech coach made videotapes of him and pointed out the flaws. The glasses came off, replaced by soft contact lenses. A new barber sheared off the sideburns and closely cropped his silvery hair. He began jogging and lost weight. He was made to dress in well-cut business blues, blacks and grays.

The finishing touch was the red tie. It is the only color of tie he wears in public now, and practically everyone in the province who wears a red tie is taken for one of his supporters.

Trudeau's former operatives also helped Peterson in the delicate process of stepping agilely from the right of the political center to the left of it.

Completing the picture were his three small children and an actress wife, Shelley.

The new Peterson has retained a certain humility and the occasional awkwardness of his former self, and voters here seem to find that redeeming. He still stutters when he is without a script, and he gets into trouble occasionally with attempts at humor.

For example, in a recent campaign speech before a Toronto Rotary Club, Peterson announced a new preventive health program to promote "wellness" and then turned to the club president, an undertaker, and said, "You may not like that in your business."

During a recent debate, his two opponents tried to pin him down on a proposed free-trade deal with the United States. One called Peterson "wishy-washy"; the other taunted him to show some "gumption." Peterson acknowledged that he had been bested in the debate, and public opinion polls agreed. But he said he had not been nervous. "I stumble all the time," he said.

But neither the jokes nor the poor debate showing appear to have hurt him or affected his lead. Indeed, indications are that voters are reassured by his foibles and like his modesty.

"Being very bright doesn't make you loved or politically saleable" in Ontario, said Desmond Morton, a University of Toronto political scientist. In Canada's richest and largest province, with more than one-third of the nation's population, voters prefer "men of the center with a right-wing look about them," Morton said.

Peterson is a millionaire from London, Ontario, who ran his family's successful electronics business before going into politics. His big break came in mid-1985 when he agreed to an unorthodox political accord with the leftist, union-oriented New Democratic Party.

A few months earlier, Bill Davis, the long-governing, popular provincial Progressive Conservative Party premier, had suddenly announced his retirement, and an election was called. The Conservatives picked a businessman to be their candidate, but he scared many Ontarians by talking about experimenting with Reaganomics. The Conservatives won 51 seats, less than a majority and not enough to form a government. Peterson's Liberals picked up 49 seats, and the New Democratic Party, led by Bob Rae, got 25.

Rae shrewdly negotiated with both the Liberals and Conservatives before agreeing to give Peterson the backing of his New Democrats on major issues while reserving the right to openly criticize him and the Liberal government.

In exchange, Peterson agreed not to call an election for two years and promised to implement a number of New Democratic programs.

Living up to his end of the bargain, Peterson forced physicians to stop their practice of "extra billing" patients above the amount reimbursed by the government's health insurance plan. He also pushed through legislation enacting a "comparable worth" pay system for women in government, changed the patronage system and provided full public financing to the province's Roman Catholic schools.

Many observers feel that most Ontarians liked the hybrid arrangement.

"A lot of people in Ontario are convinced that the best government we can get are minority governments," said Canadian Broadcasting Corp. political reporter Gerry McAuliffe. "Canadians like to have someone watching the party in power so that it does not get too arrogant."

This summer, the two years were up, and Peterson called an election. Every published poll indicates that he has the support of nearly half the electorate, enough to give him a large majority of seats.

Peterson's two years as premier have coincided with a boom in the province that has led Canada into the strongest economic expansion of all the major industrial countries, and he has won high praise. The national magazine Saturday Night described Peterson as "the premier who walks on water."

The once thrifty businessman who used to urge restraints on government spending cheerfully hands out government grants as he roams the province, pressing the flesh, giving soul handshakes to black teen-agers, kissing babies and elderly women.

Going into the campaign with a budget surplus of more than $750 million, Peterson has announced new spending programs that include wheelchairs and artificial limbs for the disabled poor, tax credits for middle-income wage earners saving to buy their first homes, start-up government grants of up to $30,000 for young farmers and more teachers and 40,000 new computers for elementary schools.

The Conservatives, who now have their third leader in two years -- Larry Grossman, a veteran Toronto member of the provincial legislature -- are running third. Rae is in second place.

No premier of any of Canada's 10 provinces has ever become prime minister. Analysts and politicians say this is because those who are popular in their home provinces are regarded elsewhere in the country as too partisan to regional interests. Peterson may also face this dilemma. There is growing sentiment against the initiative to negotiate a trade deal with the United States in Ontario, but there is support for it in the resource-based western and Atlantic coast provinces.

But some feel the rules could be rewritten because of a vacuum in the national political scene. Conservative Prime Minister Brian Mulroney has languished at the bottom of the polls for months, with only about 20 percent of the electorate giving him passing marks.

John Turner, leader of the federal Liberal Party, does not fare much better. In fact, rebels in the Liberal parliamentary caucus are hoping to overthrow him less than a year after the party had reaffirmed his leadership.

Federal New Democratic Party leader Ed Broadbent is much respected, and his party swept all three by-elections this summer. But many are skeptical that he can overcome reluctance about the party's mildly socialist philosophy to win enough seats to form a government.

There are also no bright lights among Peterson's fellow provincial premiers. New Brunswick's Richard B. Hatfield is best known, but mostly because Royal Canadian Mounted Police seized small bags of marijuana that were stowed in his luggage as he boarded a plane for a meeting with Queen Elizabeth II.