Former U.S. President Barack Obama on Thursday offered his support in upcoming Canadian legislative elections for Justin Trudeau, the prime minister with whom he has been chummy for years.

"Wishing my friend @JustinTrudeau the best in Canada's upcoming election," Obama said in a Twitter message, just four days before Canadians go to the polls.

"Justin has been an effective leader and strong voice for democratic values, and I'm proud of the work we did together," he said.

The message was similar to one sent by Obama in October 2019 when he lauded Trudeau as a progressive statesman the world needs and urged Canadians to back him for another term.

Trudeau was back then, as he is now, in a tight re-election race against a rookie Tory leader.

Obama and Trudeau first met as leaders of their respective countries at an Asia-Pacific Economic Cooperation (APEC) summit in November 2015, one month after Trudeau won his first general election.

Four months later, Trudeau became the first Canadian leader in 20 years to make an official pomp-filled visit to the White House.

The warmth of the encounter triggered a flood of online jokes and tongue-in-cheek reports of a budding "bromance" between the two men.

Obama saluted Trudeau as his political and generational kin, even though the American is 10 years older. Both have championed the importance of issues like health care, gay rights and climate change.

Since then the pair have been spotted hugging goodbye outside of Trudeau's favourite Montreal restaurant after a meal, and sharing a beer when Obama was in Ottawa to deliver a speech.

On Thursday, Trudeau tweeted back: "Thanks for your support, @BarackObama. Progress is on the ballot -- and we're going to keep fighting for it."