The current debate over immigration reform among some members of the Republican presidential field has been barreling toward the extreme for months now. On Sunday, Wisconsin Gov. Scott Walker may have finally pushed it over the line.

In an interview with NBC’s Meet the Press host Chuck Todd, he spoke favorably about building a wall across the border – the border with Canada.

Related: Scott Walker Adjusts his Immigration Stance. Again

Walker’s comments came in the context of a discussion about border security. Todd pointed out that many in the GOP field, Walker included, say border security needs to be increased not just to prevent the crossing of illegal immigrants looking for work, but also to stop terrorists from crossing the border. If that’s the concern, he said, why is there so much talk about walling off Mexico, but comparatively little about walling off Canada.

Todd hadn’t even finished the question before Walker jumped in, asserting that he has, in fact, been talking about a wall on the northern border.

“Some people have asked us about that in New Hampshire,” he said. “They’ve 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.”

The line separating the U.S. and Canada is the longest undefended border in the world. It separates two countries that have, for centuries, been friends, staunch allies, and each other’s largest trading partners.

Related: Bush Warns that Trump’s ‘Unrealistic’ Wall Will Cost Hundreds of Billions

To be sure, the security on the Canadian border – not long ago consisting of traffic cones left in the road overnight when crossing points were unmanned – has been substantially beefed up in the years post-9/11, as this piece in Mother Jones documents.

But a wall?

There was a time when any politician running for office in the U.S. would have found the idea of putting a defensive wall between the U.S. and Canada laughable. We don’t live in that time anymore.

Today, we have loudmouthed billionaire Donald Trump leading the polling for the Republican presidential primary, and a huge pack of other contenders nipping at his heels, trying desperately to attract some media attention when the cameras aren’t all trained on him.

Trump calls for a wall on the southern border? Most of the other candidates quickly fall in line.

Trump promises to physically deport 11 or 12 million undocumented immigrants? A few other candidates quickly suggest that they are open to the idea.

Related: Are Immigrants Really Taking Americans’ Jobs?

Trump calls for the abolition of constitutionally guaranteed birthright citizenship? Other candidates start jumping on the bandwagon.

Perhaps the GOP race is at the point where the only way for candidates to break free of Trump’s gravitational pull is to start suggesting things even more outrageous than he does.

Walker showed signs of this last week, calling for President Obama to cancel a scheduled state dinner with Chinese President Xi Jinping over what he suggested was China’s role in last week’s stock market decline.

Buckle up, people. We’re still 10 months away from the Republican nomination convention next year. If we’re already walling ourselves off from Canada at this point, it’s anybody’s guess what proposals will get aired between now and then.