The Republican presidential contender said the idea of a northern border has come up while he has campaigned in New Hampshire.
In an interview for NBC's "Meet the Press" available online, Walker said his tough talk to securing the borders and enforcing U.S. laws extends to the 5,525-mile Canadian border as well.
It's a shift from most campaign-trail rhetoric, which has focused on the United States' border with Mexico, where millions undocumented immigrants have entered the country.
"Some people have asked us about that in New Hampshire," Walker said. "They raised some very legitimate concerns, including some law enforcement folks that brought that up to me at one of our town hall meetings about a week and a half ago. So that is a legitimate issue for us to look at."
Terrorist attacks have been plotted in Canada -- including the so-called "Millennium plot," a foiled 2000 plan in which an Algerian national planned to cross into the United States from Canada and bomb the Los Angeles International Airport.
Democratic Sen. Patrick Leahy, of Vermont, slammed the comments on Monday, saying that having grown up near the Canadian border, Walker's suggestion is to him "one of the craziest" ideas to come out of the election.
"Gov. Walker simply must be unaware of the economic prosperity that commerce across the northern border brings to the United States," Leahy said in a statement. "Those of us who represent states that share a border with Canada know better. ... It is disappointing but not surprising that yet another Republican presidential candidate is using the border to score cheap political points."
Walker has focused his immigration remarks on enforcing U.S. laws already on the books in recent weeks.
He ran into trouble two weeks ago after GOP front-runner Donald Trump proposed repealing the Fourteenth Amendment's mandate that children born in the United States automatically become citizens, regardless of their parents' legal status.
Walker first said he favored Trump's idea of repealing birthright citizenship. He backed off that stance days later, telling a reporter that he hasn't taken a position. And then last Sunday, he said that he isn't advocating any changes to laws on the books -- including the Fourteenth Amendment.
Despite taking three different positions
in the course of a week, Walker insisted Sunday on NBC, his stance has been consistent.