Wisconsin Gov. Scott Walker, a Republican presidential candidate, speaks in Brooklyn Center, Minn., on Aug. 18. (Jim Mone?AP)

Wisconsin Gov. Scott Walker (R) says the United States needs to secure all its borders — and that includes reviewing security on the far-less-controversial northern border it shares with Canada, the longest border in the world at 5,525 miles.

Walker has long said that securing U.S. borders, especially the southern border with Mexico, is not only a deterrent to illegal immigration but also a way to ensure that terrorists and international criminals do not enter the country. During an interview on NBC News's "Meet the Press" that aired Sunday morning, Walker pointed out that the United States spends millions of dollars on security at airports and ports and that it "only makes sense" to also pay that level of attention to the borders, including the one with Canada.

Moderator Chuck Todd asked Walker  whether he would support building a wall along the long northern border.

"Some people have asked us about that in New Hampshire," Walker responded. "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's a legitimate issue for us to look at."

[Scott Walker tells undocumented worker that he must follow the law]

Walker added that securing the borders means not just building a wall but also ensuring that the "intelligence community has the ability for counterterrorism and the ability to go after the infrastructure they need to protect us."

Although immigration has long been a key issue for Republicans as they pick their presidential nominee, Donald Trump — the bombastic businessman and television celebrity who is now the GOP front-runner — has blown up that discussion by pushing a controversial and pricey proposal under which all illegal immigrants would be deported, a massive wall would be constructed along the southern border and U.S.-born children of illegal immigrants would stop getting automatic U.S. citizenship. It has been difficult for other candidates to distinguish themselves on this issue, although several are trying.

New Jersey Gov. Chris Christie (R) said during a campaign event in New Hampshire on Saturday that the United States should track foreign visitors just like FedEx tracks packages — and that, as president, he would ask Fred Smith, the founder of FedEx, to work for the government for three months to set up such a system.

[Never mind the wall. Christie says foreign visitors should be tracked like FedEx packages.]

In an interview with ABC's "This Week" that aired Sunday, Louisiana Gov. Bobby Jindal (R) said immigrants who come to the country legally should immediately "learn English, adopt our values, roll up our sleeves and get to work." Jindal is the son of immigrants from India and has called for an end to "hyphenated Americans."