Prime Minister Brian Mulroney is pinning his re-election hopes on Canadians supporting free trade with the United States, but the country's sour mood over sprinter Ben Johnson's disgrace in the Olympics is overshadowing the Conservatives' key election plank.

Even before Mulroney dissolved Parliament Saturday and scheduled elections Nov. 21, Canadians were unenthusiastic about parliamentary elections because none of the country's three parties has much popular support.Now, they will endure a 51-day campaign, disgruntled over Johnson's rapid fall from grace after first winning the Olympic 100-meter gold medal and being honored as the world's fastest human, then being stripped of the honor because he tested positive for steroid use. Canadian pride went from boom to bust overnight.

Mulroney had planned the election as a referendum on the free trade pact he signed earlier this year with President Reagan. The Senate, dominated by the opposition Liberal Party, has balked at approving it unless an election demonstrates public support. Polls show Canadians evenly divided on the issue.

The Canadian leader had hoped Johnson's gold medal would rub off on him, helping to take the tarnish off his own four-year administration, which has been marred by scandals.

Instead, his call for elections was greeted with somber indifference amid the national angst over Johnson.

The day after Mulroney announced the election, the capital's Ottawa Sunday Sun newspaper committed the first two pages to an interview with Johnson. The election call was on page 5.

Mulroney is under pressure to hold an immediate judicial inquiry into the Olympic scandal, giving Johnson an opportunity to clear himself of knowingly taking drugs.

But such an inquiry during the campaign could embarrass the government. Sports Minister Jean Charest, who last week banned Johnson for life from competing for Canada, has acknowledged that he twice ignored warnings that Johnson was using banned drugs.

Chris Kelly, spokesman for the Canadian Track and Field Association, acknowledged that the Johnson issue is being used by the two opposition parties, but "that's their perogative.

"There's a sports program of the federal government that uses taxpayers' money. I don't like to see politics take over sports, but obviously with our Canadian sports system, it's necessary."

Mulroney's race to win a second term for his Conservative Party is likely to be more successful than Johnson's steroid-fueled dash, but only because the Liberals and socialist New Democratic Party hold even less appeal for Canadian voters than his Tories.

Johnson's sweet-and-sour Olympic experience was a fitting conclusion to the Mulroney administration. It was highlighted by the historic free trade agreement and four years of economic prosperity, but was marred by a series of scandals, ministerial resignations and patronage appointments.