OTTAWA — Prime Minister Justin Trudeau’s political gamble failed to pay off Monday when Canadian voters returned him to office but denied him the expanded bloc of power he had been seeking in Parliament.

Unofficial election results on Monday indicated that while he would remain as prime minister, it would again be as the head of a minority government.

In August, with his approval ratings high, Mr. Trudeau called a “snap election,” summoning voters to the polls two years before he had to. The goal, he said, was to obtain a strong mandate for his Liberal Party to lead the nation out of the pandemic and into recovery.

But many Canadians suspected that his true ambitions were mere political opportunism, and that he was trying to regain the parliamentary majority the Liberals had until they lost seats in the 2019 election.

Whatever his motive, it did not work.

With some votes still being cast or uncounted, the preliminary results were a near repeat of the previous vote. The Liberal Party won 156 seats on Monday — one fewer than it acquired in 2019 — while its main rival, the Conservative Party, won 121 seats, the same as before.

“If you missed the 2019 election, don’t worry, we just did a rerun for you,” said Duane Bratt, a political scientist at Mount Royal University in Calgary, Alberta.

The outcome left Mr. Trudeau in a familiar situation. To pass any laws, he will once again have to win members of the opposition over to his side. And, at least in theory, his party’s shaky grip on power leaves his government vulnerable to being overturned by Parliament.

In his victory speech early Tuesday, Mr. Trudeau acknowledged the unpopularity of his call for a snap election.

“You don’t want us talking about politics or elections anymore; you want us to focus on the work that we have to do for you,” he told a partisan crowd in a hotel in downtown Montreal. “You just want to get back to the things you love, not worry about this pandemic, or about an election.”

In calling for the early election, Mr. Trudeau had argued that, like his predecessors in the aftermath of World War II, he needed a strong mandate from voters to vanquish the coronavirus and rebuild the national economy, badly damaged by the pandemic.

But the announcement was not well received by many Canadians.

Alarm that the government was holding an election when it did not have to, even as the Delta variant was straining hospitals in some areas, never abated for many voters during the 36-day campaign. And Mr. Trudeau’s opponents were quick to characterize his move as a reckless power grab. Erin O’Toole, the Conservative leader, went so far as to call it “un-Canadian.”

In the end, Mr. Trudeau not only failed to secure a majority in Parliament, according to unofficial results, he may have also squandered the good will he had gained as he led his nation through the coronavirus crisis.

“I’m wondering if the Liberals, in their minds, are saying: ‘Dang it, why did we — why did we call it?’” Kimberly Speers, a professor of political science at the University of Victoria in British Columbia, said during the final week of campaigning.

Now, she said, it is unclear how long any Liberal minority government will be able to hold together and what this will all mean for the party’s leader. “How long is Trudeau going to last?” Ms. Speers wondered.

When Mr. Trudeau first ran for office as leader of the Liberals in 2015, few political experts thought he could pull it off. He began that campaign in third place, behind the incumbent Conservatives and the left-of-center New Democratic Party.

He won by presenting himself as a new voice in politics with a different approach and different ideas to go with it

But that fresh young politician was little to be seen this time around.

Mr. Trudeau, 49, offered voters less a vision for the future than a warning, sometimes explicitly. A return to the Conservative government under Mr. O’Toole, he said, would wipe away his government’s achievements in a variety of areas, among them gun control, gender equity, climate change, child care, poverty reduction and, above all, fighting the pandemic and getting Canadians vaccinated.

“Mr. O’Toole won’t make sure the traveler sitting beside you and your kids on a train or a plane is vaccinated,” he said at a campaign rally in Surrey, British Columbia, last week. “This is the moment for real leadership. Mr. O’Toole doesn’t lead — he misleads.”

But in Mr. O’Toole, the prime minister was running against a different opponent than the Conservative leaders he had encountered in the two previous campaigns.

“I am a new leader with a new style,” Mr. O’Toole, who took over the party just over a year ago, said at the outset of the campaign. “There are five parties but two choices: Canada’s Conservatives or more of the same.”