Donald Trump said during the latest Republican debate that he sees no contradiction in wanting to build a wall between the U.S. and Mexico, but not along the Canadian border.

Trump doubled down on his desire if elected president to build a wall along the Mexico border at the CNN-Telemundo debate taking place at the University of Houston on Thursday night. Because Mexico officials had derided the idea in recent days, Trump said, "the wall just got 10 feet taller."

Maria Celeste Arraras, a Telemundo journalist, challenged Trump on whether he was focused on the right border, given that ISIS has called upon its supporters to launch terrorist attacks in Canada.

"As a matter of fact, U.S. officials have warned that it is the Canadian border which is the most significant threat," she added.

Trump responded that the relative practicality and threat level made the idea a non-starter.

"The problem with Canada, you're talking about a massively long piece," said Trump. "You're talking about a border that would be about four times longer. It would be very, very hard to do, and we – it is not our biggest problem. I don't care what anyone says. It is not our big problem.

"Our big problem [with Mexico] is not only people coming in, and in many cases the wrong people, it's the tremendous amount of drugs that are coming in."

The realistic political prospects of building a wall at either border aside, it's a stance that Trump previously expressed to CBC reporter Meagan Fitzpatrick on the campaign trail last year.

"I love Canada," he told Fitzpatrick in Washington in September. "I would not build a wall on the Canadian border."

Homeland security meeting

For her part, Arraras was likely referring to a meeting earlier this month of the U.S. Senate homeland security committee, during which witnesses and senators expressed concern about the number of Syrian refugees Canada was admitting, as well as the number of agents who patrol the shared border.

Successive officials from the National Border Patrol Council have argued over the years for a beefed-up number of agents on the border, and that Canada is more of a threat when it comes to the movement of would-be terrorists into the U.S. than Mexico.

They would likely point to the conviction last year of two men for a plot to derail a Via Rail train travelling between New York and Toronto. One of the men, Raed Jaser, originally entered Canada with a fake passport and stayed for years even after his refugee claim was denied. Before the 9-11 attacks, Montreal native Ahmed Ressam made his way to B.C. to launch a planned attack on the Los Angeles airport, but was apprehended in Washington state.

As for attacks north of the border, Dabiq, the magazine published by the Islamic State, has advocated attacking Canada on multiple occasions due to the country's involvement in the anti-ISIS campaign in Syria and Iraq.

Did Trump just say could build wall on US-Canada border because it is 4x US-Mexico border? US-Canada is 1,538 miles; US-Mexico 1,933 miles. —@KarlRove

The idea of a wall between Canada and the United States held currency in the Republican race for Wisconsin Gov. Scott Walker, but he has long since dropped out.

Ben Carson, still in the race, proposed the U.S. military and National Guard patrol the U.S.-Canada border in his "Seven Steps to a Safer America" plan.

None of the other four candidates taking part Thursday were asked to weigh in on the Canadian question, with the debate quickly changing focus to another topic.

Longtime Republican operative and Fox News commentator Karl Rove was forced to apologize for a tweet during the debate that incorrectly posited that the southern and northern borders the U.S. shares are of comparable length, although Trump's math was also erroneous – albeit closer to the mark.

The U.S.-Mexico border is over 3,100 kilometres long. The border between Canada and the U.S. which features the 49th parallel comprises over 6,400 kilometres, with an additional 2,475 kilometres bordering Alaska.