OTTAWA (REUTERS) - Canadian Prime Minister Justin Trudeau, facing potential defeat in a Sept 20 election, on Thursday (Sept 9) used a key leaders' debate to take aim at his main rival, portraying him as weak and ineffective.

Polls show the Conservatives of Mr Erin O'Toole have a chance of winning the election and ending six years of Liberal rule.

Mr Trudeau called the vote two years early as a referendum on his handling of the Covid-19 pandemic.

Mr Trudeau, sometimes looking agitated, rounded several times on Mr O'Toole, who has had trouble making himself known to Canadians since taking over his party a year ago.

Mr O'Toole said he will offer serious leadership to clean up after what he called a corrupt, incompetent and spendthrift Trudeau government.

Mr Trudeau accused Mr O'Toole of harbouring an extremist agenda and not being serious on topics such as climate change.

Mr Trudeau also favoured mandates to ensure people were inoculated against Covid-19, a move Mr O'Toole said went too far.

"The problem with Mr O'Toole and his principles is he says all the right-sounding things and he's working on reassuring everyone that he's right there as a strong leader, but he can't convince his candidates to get vaccinated," said Mr Trudeau.

Polls show Mr O'Toole with a slight lead amid voter unhappiness with Mr Trudeau's decision to call an election two years early.

The debate is the only one of three in English, spoken by two-thirds of Canada's 38 million people, and is traditionally seen as a key means of influencing voters.

However, Nanos Research pollster Nik Nanos said by email "there were no major gaffes nor any knock-out punches from any of the parties... this wasn't a game changer".

Mr Trudeau spoke over the other four party chiefs several times, forcing the moderator to cut him off.

Mr Darrell Bricker, chief executive officer of Ipsos Public Affairs, said he did not see anything from Nr Trudeau or Mr O'Toole that would change the direction of their campaigns.

"When he (Trudeau) did try to go at O'Toole it came off as very hot and frantic. O'Toole wasn't a huge factor tonight but that's OK," he said by email.

Mr Trudeau is fond of noting that earlier this year, most Conservative lawmakers voted in favour of draft legislation that would have banned some abortions. The initiative failed.

Mr O'Toole insisted he was in charge of his party and would not bow to the views of legislators with hardline social views.

"I am driving the bus to make sure we get this country back on track. And I'm here to defend the rights of all Canadians, women, members of the LGBTQ (lesbian, gay, bisexual, transgender and queer) community," he said.

Mr O'Toole conceded that in the past, the Conservatives had not done enough to combat emissions of greenhouse gases.

"This is an area where the Conservatives had to win back some trust. We hadn't met the expectations of Canadians on climate change," he said.

Before the debate, Mr O'Toole received a boost when Quebec Premier François Legault said a vote for the Conservatives would be better than one for the Liberals.

Quebec accounts for 78 of the 338 seats in the House of Commons and is crucial for any party seeking office.

A three-day rolling Ekos phone poll of 1,365 adults released on Thursday showed the Conservatives at 33.6 per cent public support versus 30.7 per cent for the Liberals and 15.7 per cent for the smaller left-leaning New Democrats.

The poll had a margin of error of 2.7 percentage points, 19 times out of 20.