WASHINGTON (BLOOMBERG, REUTERS) - Former US President Barack Obama endorsed Justin Trudeau in his re-election bid, with polls showing the Canadian prime minister locked in a tight race with his Conservative rival.

"I was proud to work with Justin Trudeau as President. He's a hard-working, effective leader who takes on big issues like climate change," Obama tweeted on Wednesday (Oct 16).

"The world needs his progressive leadership now, and I hope our neighbors to the north support him for another term."

Trudeau is seeking a second term for his Liberal government in Monday's election.

Most opinion surveys show the Liberals tied with the opposition Conservatives, led by Andrew Scheer, though one pollster says Trudeau has an inside track to win more seats.

The election appears set to produce a minority parliament, meaning that either Trudeau or Scheer will need support from lawmakers in smaller parties to pass laws.

Obama found common cause with Trudeau on climate change before leaving office.

He welcomed the Canadian leader to the White House for a state visit in March 2016.

This is not the first time Obama has endorsed a foreign leader. He tweeted his support for Emmanuel Macron ahead of France's 2017 presidential election.

The American embassy in Ottawa declined to comment on the endorsement on the grounds that Obama is a private citizen.

The Liberals, the Conservatives and the New Democrats - the three main parties contesting the vote - did not immediately respond to requests for comment.

Elections Canada, the country's nonpartisan elections administrator, said Obama's tweet did not contravene regulations on foreign interference.

"It's free speech. The only issue would be if Mr Obama started to spend money on Mr. Trudeau's campaign," spokesman Pierre Cadieux said by phone.