When Donald Trump talks about immigration, at the core of his policy proposal is the construction of a wall along the U.S.-Mexico border.

But what about to the north along the Canadian border? Some of his GOP challengers say it's a legitimate idea.

"People have asked us about that in New Hampshire," Wisconsin Gov. Scott Walker said on NBC's "Meet the Press." "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."

Walker's comments come as Trump has been assailed for his immigration plan that calls for a wall to be built along the southern border which, says the business mogul, would be paid for by the Mexican government. Critics have called the proposal unrealistic.

Moreover, Trump's plan also calls for an end to birthright citizenship, which has caused some heartburn for Walker as he's wavered. Initially he said he supported changing the U.S. Constitution and ending birthright citizenship, before then backing away from the comment.

Read more Times coverage about Trump, the 2016 GOP field and immigration.

Donald Trump was absent from the Sunday talk shows for the first time in weeks. But those Republican hopefuls who did sit down for network interviews still had to grapple with the surprise Republican front-runner.

Perhaps no candidate has suffered because of the Trump surge more than Wisconsin Gov. Scott Walker, who was asked on NBC's “Meet The Press� about what he could do to regain momentum he was seeing before the real estate magnate took off.

“It's hard work. It's going out. It's doing town hall meetings,� he said. “For us, I think the biggest spark for us is getting the message out that now is not the time to put in place someone who hasn't been tested before.�

Republican Louisiana Gov. Bobby Jindal argued in a pair of interviews that as voters began taking their choice more seriously after the summer months, the landscape will change.

“I think after we get past this summer of silliness and insults, the voters are going to begin to look at who is prepared to do the job,� he said on ABC's “This Week.� “What I see is that voters haven't committed to any candidate yet. In Iowa, in these early states, they're kicking the tires, they're asking the tough questions. This is a wide open race.�

New Jersey Gov. Chris Christie insisted on “Fox News Sunday� that Trump's standing was a reflection of the crowded field, and that he would continue to focus on substantive policy proposals. He specifically dismissed Trump's immigration plan as “simplistic.�

All three also faced questions about one of the issues Trump has focused on most: immigration.

Christie defended saying that the government should use technology to track illegal immigrants as companies like FedEx do to track packages.

“My point was this is once again a situation where the private sector laps us in the government with the use of technology,� he said. “We should bring in the folks from FedEx to use the technology to be able to do it. There's nothing wrong with that. And I don't mean people are packages. So, let's not be ridiculous.�

Walker, meanwhile, said he's even heard voters in New Hampshire advocating tougher immigration enforcement at the Canadian border.

“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,� he said of building a wall on the U.S.-Canada border.

Jindal said the U.S. has a “dumb� immigration policy now.

“We need to insist that folks who come here come here legally, learn English, adopt our values, roll up our sleeves and get to work,� he said. “Immigration without integration is not immigration; it's invasion. My parents are proud of their Indian heritage, but they came here to be Americans and they love this country.�

As a new poll shows him within striking distance of Hillary Rodham Clinton in the key state of Iowa, Vermont Sen. Bernie Sanders argued Sunday that his success is a reflection of his message, and not uneasiness in the Democratic Party over its putative front-runner.

“This is not anti-Hillary Clinton. This is pro-Bernie Sanders, and pro a message that says enough is enough,� Sanders said in an interview on ABC's “This Week.� “This country and our government belong to all of us, not just a handful of very wealthy people.�I In another interview on CNN's “State of the Union,� Sanders ticked off a number of key policy differences he has with Clinton, including banking reform, trade, drilling for fossil fuels, a national $15 minimum wage and expanding Social Security.

And while he praised Barack Obama's 2008 campaign as “brilliant,� he faulted the president for failing to enlist the public for support needed to see through many of his promises, and instead trying to negotiate with congressional Republicans.

“That is what I'm talking about when I speak about a political revolution,� Sanders said. “It can't be done within the Beltway itself. We need a mass movement, and that's what we are trying to create, and are succeeding in creating right now.�

A new Des Moines Register/Bloomberg poll shows that Sanders trails Clinton by just 7 percentage points among likely Democratic caucus-goers, 37%-30%. A Bloomberg Iowa poll in mid-June showed Clinton with a 50%-24% advantage over Sanders in the first caucus state.

A new factor in the poll is Vice President Joe Biden, who has not announced his 2016 plans but is giving it more serious thought as he continues to deal with the death of his son, Beau. Biden earned 14% support in the new Bloomberg poll.

Sen. Amy Klobuchar (D-Minn.), who has endorsed Clinton, said in a separate interview on ABC that Biden is beloved among Democrats, while also pointing to what she said is strong support for the former secretary of State.

“I think you have heard from her directly from the very beginning that this is not a coronation,� Klobuchar said. “You can't just waltz in and win a Democratic primary. We've seen many people in the past think they could do that, and that's not what happened.�

Klobuchar said she “appreciated� the new tone from Clinton as she addresses concerns about her use of a personal email account, insisting that voters were more concerned about bread-and-butter economic concerns.“ She is going to be testifying at a public hearing in October. Her arch-enemies in the House of Representatives can ask what questions they want,� Klobuchar said. “When I'm at the Minnesota State Fair, what people are talking about are their jobs, they're talking about can they send their kids to college.�

A new poll released Saturday showed Vermont Sen. Bernie Sanders trailing Hillary Rodham Clinton by single digits in Iowa, about five months before caucus-goers are set to cast ballots in the presidential race.

Clinton, who remains a strong frontrunner to secure the Democratic nomination, holds a seven-percentage-point advantage over Sanders, a self-described socialist who has tapped into support from the liberal base of the party.

The poll, released by the Des Moines Register/Bloomberg Politics, reveals that support for Clinton has slowly trickled downward since her campaign kickoff in April. It's the first time Clinton has fallen below 50% in the four polls conducted by Register/Bloomberg this year.

Here is a snapshot of the most recent poll, which has a margin of error of plus or minus 4.9 percentage points.

Hillary Clinton: 37%

Bernie Sanders: 30%

Joe Biden: 14%

Martin O'Malley 3%

Jim Webb: 2%

Lincoln Chafee: 1%