"Justin Trudeau has been dividing Canadians - pitting regions of our great country against each other", Simon Jefferies, a spokesman for Scheer's campaign, said in a statement, adding Trudeau "should take a look in the mirror".
"The world needs his progressive leadership now, and I hope our neighbors to the north support him for another term", he wrote on Twitter.
Trudeau was the first Canadian prime minister the United States honoured with a state dinner in nearly two decades when Obama invited him and his wife to Washington in March 2016 after Trudeau won power the previous October.
Obama's endorsement of a former blackface-wearing Canadian prime minister highlights a contrast with Obama's handling of US presidential primary, in which he has not publicly endorsed any candidate.
With the Canadian election taking place on October 21, the campaign is coming down to the wire. He called Trudeau a "hard-working, effective leader" who has taken on major issues like climate change.
Before that speech, Trudeau and Obama shared a beer at a brewery.
Obama's endorsement was also surprising because of his decision to engage in global politics, choosing to meddle in a foreign election.
During Obama's presidency, the Canadian and American leaders formed a close bromance, united by progressive policies on issues such as climate change and women's rights.
Trudeau's Liberal Party posted Obama's message into a party fundraising pitch that was e-mailed directly to potential donors.
Mr Trudeau's rival, Conservative leader Andrew Scheer, said that he is "not very interested what former foreign leaders are saying".
In 2017, he voiced his support in a video for French presidential candidate Emmanuel Macron.
For some, Obama's endorsement of Trudeau raised concerns.
Scheer said that he would let Canadians judge whether the former U.S. president's endorsement is appropriate.