TORONTO, SEPT. 10 -- Ontario Premier David Peterson, who promised worried voters that he will protect their jobs in the free-trade negotiations between Canada and the United States, led his Liberal Party to a landslide victory today.

It was the first time the Liberals won a majority in an Ontario election in 50 years and is certain to boost the chances of millionaire Peterson becoming a serious contender to be prime minister of Canada.

Widespread anxieties about jobs pushed the trade issue to the top of the agenda in the short summer election campaign. Peterson, who had previously taken a calculatedly ambiguous position on the issue, was forced during the campaign to assure brewers, farmers, magazine publishers, auto workers and many others that he would veto any pact that did not provide adequate safeguards for them.

His opposition to an agreement, a draft of which is to be made public by the first week of October, could wreck any deal, especially given the size of his victory tonight in Canada's richest and most populous province.

On the basis of totals registered before midnight, analysts predicted that the Liberals would take 95 of the 130 seats in the provincial legislature with at least 50 percent of the vote. The leader of the Conservative Party, Larry Grossman, lost in his own district to a Liberal. Bob Rae, leader of the left-wing New Democratic Party, narrowly retained his seat.

Analysts were predicting this evening that the New Democrats eventually would win 19 seats, with the Conservatives holding on to only 16.

Peterson's Liberals came in second in the last election, two years ago, but since none of the three parties had a majority, he was able to form a government by forging an unusual alliance with the third-place New Democrats. That alliance forced the provincial Conservative Party out of power after more than 42 years. The Liberals last won a majority in an Ontario election in 1937.

Peterson, 43, who took speech lessons, lost weight, restyled his hair and got a new wardrobe, saw his standing soar in public opinion during the past two years. Every major opinion poll indicated his party would win today by a wide margin.

Indeed, the only suspense in the campaign was the jockeying by the other parties for second place.

Grossman, battling to keep his party from oblivion, appealed to right-wing voters. While he came out unequivocally for a free-trade deal with the United States, the more emotional issue he raised concerned language, almost always a fractious topic in Canada.

Grossman suggested that Peterson intended to make Ontario officially bilingual, causing some to worry that knowledge of both French and English would be required for employment. Clearly considering Prime Minister Brian Mulroney, his fellow Conservative, more of a liability than an asset, Grossman did not invite him to help campaign.

New Democratic Party leader Rae attempted to capitalize on his party's growing national popularity and brought in federal NDP leader Ed Broadbent, consistently shown in polls as the country's most popular leader.

Although past New Democratic platforms have called for the government to take over some resource industries and steep increases in the minimum wage, Rae played down these issues.