Sen. Rand Paul Randal (Rand) Howard PaulSecond GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill MORE (R-Ky.) is panning calls to build a wall along the nation's northern border with Canada, something GOP rival, Gov. Scott Walker (Wis.), would not rule out.

"That's a pretty dumb idea," Paul said with a laugh during an interview with Boston Herald Radio, calling a wall between the U.S. and Canada "a sort of a ridiculous notion.”

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Walker said on Sunday that he would consider building the wall, given the potential risk of terrorists crossing the northern border into the U.S.

Paul also mocked GOP front-runner Donald Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE's claims that he would build a wall along the southern border and force Mexico to pay for it.

“There have been a lot of dumb ideas put out. The one that said the Mexicans will pay for the wall is probably the dumbest of dumb ideas,” Paul said on Monday.

Mexico's government has said it won't pay for a wall.

Paul is currently lagging in the polls, sitting 10th in the 17-person GOP field in the RealClearPolitics average of recent national polls.

The Kentucky lawmaker suggested the Canadian wall was the latest attempt by Republicans looking to portray a tough image when it comes to handling illegal immigration.

“Everyone is now competing to say, ‘Oh no I will put them in camps,' 'Oh no I will throw them out,' 'I will put everyone in jail,' 'I will have an electric fence and I'll do this,’” he said.

“The biggest thing we need to do is have a functioning immigration system with a good work program, we need to say that we have zero tolerance — you come in illegally, you'll be sent back," Paul added.

“There are places for walls in some of the big cities along the southern border, but we have gotten into this shouting match about who can build a bigger wall. We have some people saying, ‘My wall is going to be as big as the Great Wall of China, how big will your wall be?’”