Also in pursuit of a bigger share of Canada's political pie are social democrat Jagmeet Singh and climate crusader Elizabeth May, who hope to deny the frontrunners a majority.
The first-born son of late Canadian prime minister Pierre Trudeau, Justin Trudeau brought panache and progressivism to Canada's highest office when he first swept to office in 2015.
Handsome and youthful, the outdoorsman and teacher led the once-mighty Liberal Party back to power -- echoing the "Trudeaumania" of his father's ascent.
The party had ruled for most of the last century, but had fallen on hard times until the younger Trudeau burst on the scene, rescuing it from third rank status.
Now older than his main rivals at 47, he enters the fray laden with baggage in the form of broken electoral promises and ethics breaches have cost him support -- especially among female, indigenous and young voters who gave him a landslide victory four years ago.
Representing a gritty, working-class neighborhood of Montreal, he is married with three children.
During his first term, he legalized cannabis, brought gender parity to the cabinet, resettled thousands of refugees and oversaw the largest military procurement in the nation's history to replace aging warships and fighter jets.
But his political agenda was often sidelined by diplomatic and trade rows with the US, China and Saudi Arabia. His climate strategy also triggered a revolt by several provinces.
And his party's expulsion of two women ministers critical of his meddling in a criminal prosecution -- one of them Canada's first indigenous attorney general -- earned him strong rebukes.
The scandal also left him open to criticism for failing to live up to a promise to do politics differently.
After working as an insurance broker, Andrew Scheer entered politics in 2004, representing a district in the prairie province of Saskatchewan.
Keenly interested in lawmaking nuances, he became Canada's youngest ever speaker of the House of Commons in 2011 at the start of the Tories' last term in office.
At 40 -- after narrowly beating a former foreign minister for the Conservative leadership in 2017 -- he is now the main challenger to Trudeau.
Going into the campaign, Scheer assailed Trudeau over ethics, deficit spending and foreign policy -- vowing a tougher stance on China over its detention of two Canadian nationals and its ban on agricultural shipments.
He also vowed to scrap a federal carbon tax as his first act of governing, if elected.
Affable but more comfortable perusing policy papers than gladhanding voters, he has been likened to his dour Conservative predecessor, former prime minister Stephen Harper, but "with a smile."
A devout Roman Catholic, the son of a nurse and a deacon, he vowed -- despite personal misgivings -- to uphold Canadians' abortion and same-sex marriage rights.
He is himself married with five children.
A former criminal defense lawyer and deputy leader of the New Democrats' Ontario wing, Jagmeet Singh, 40, has struggled to differentiate himself from Trudeau on most social issues.
In 2017, the observant Sikh became the first member of an ethnic minority to lead a major federal political party.
Although a rising star in Ontario politics, Singh waited two years before seeking a seat in parliament in a February by-election in a suburb of Vancouver.
Observers say that cost him visibility on the national stage, and it has been an uphill battle to reclaim the limelight.
Born to immigrant parents, Singh married fashion designer Gurkiran Sidhu in 2018. He has also been recognized for his own style sense and a collection of high end bicycles.
In this campaign, while continuing to target Liberal supporters, the NDP is also facing a new challenge from the Greens.
US-born Elizabeth May, 65, is an author, activist and former policy advisor to Canada's environment minister who helped create several Canadian national parks and negotiate the Montreal Protocol to protect the ozone layer.
A former head of ecological group the Sierra Club, she left environmental activism to run for public office and in 2011 became the first Green to be elected to Canada's parliament.
Despite being the lone Green MP, she often managed to sway public discourse with straight talk and views that straddle political divides.
But climate arguably remains her passion, and she was arrested at a pipeline protest in 2018 for civil contempt.
Under her stewardship, the Green Party doubled its representation in parliament in a May by-election to two, and a record 15 legislators were elected to provincial assemblies.
She has a daughter with a former partner and in April married John Kidder, the brother of the late actress Margot Kidder. Both husband and wife are running for Vancouver Island seats.